Block of polyspermy

BLOCKS OF POLYSPERMY (DEVELOPMENTAL BIOLOGY )

Two mechanisms are used by animals to ensure that only one sperm will fertilize the egg to prevent the polyspermy condition .
1.FAST BLOCK - ( In sea urchins and Frogs) No fast block polyspermy are identified in mammals .

 (It happens immediately after fertilization )Depolarization of membrane so it is the change in potential of the plasma membrane. Sperm binding causes Na+ influx. 

STEPS-   a. Opening of Na+ channels in the egg plasma membrane .

                          b.Flow of Na+ into the egg cell(Na+ influx) .

                          c.Depolarization of the membrane .

                          d. As a result of that it prevents the additional sperm entry to the egg plasma membrane .

                          e. The egg plasma membrane restored to its normal -70 Mv resting potential within minutes of fusion as the Na+ channels  close  and Na+ is pumped out .

          (IF DEPOLARIZATION IS PREVENTED IT WOULD RESULTS IN POLYSPERMY )

2.SLOW BLOCK -(CHEMICAL AND MECHANICAL BLOCK)
Upon sperm entry the cortical granules fuse with the egg plasma membrane and  exocytosis of cortical granules occurs which  releases zonal inhibiting proteins like  serine proteases(which digest the connection between vitelline membrane and plasma membrane .) ,mucopolysaccharides ( It produce osmotic gradient and water rushes to perivitelline space),peroxidases ( oxidizes and cross links tyrosine )going to harden the fertilization envelope and in case of Humans it is zona Pellucida ) andhyaline which provides support during cleavage by formimg a coat around the egg called hyaline layer  .
STEPS - (In most of the species)

Inositol triphosphate -(IP3) causes the release of  Ca2+ from intracellular stores in the egg Endoplasmic Reticulum .

At the site of sperm entry ca2+ is released and it passes to the egg .

This ca2+ results in the fusion of cortical vesices with the egg plasma membrane releasing their contents into the space surrounding the egg called the perivitelline space. which is presnt in between cell membrane and vitelline envelope .

This raises the vitelline envelope and inactivates or destroys  the bindin receptor.

So, any additional sperm cannot bind which in turn prevents polyspermy .  Image source credit -http://slideplayer.com/slide/6364295/

NOTE -SOME ORGANISMS DO NOT BLOCK POLYSPERMY MANY SPERM ENTER ONLY ONE SURVIVE AND THE REST ARE DEGRADED.

What would be the reason to block polyspermy ?
Ans.       It leads to polyploidy and eventually death ..

Metamorphosis in insect

Metamorphosis in Insects (With Diagram)

The following points highlight the three main types of metamorphosis in insects. The types are: 1. Ametabolic Metamorphosis 2. Heterometabolic Metamorphosis 3. Holometabolic Metamorphosis.

Type # 1. Ametabolic Metamorphosis:

In lower insects (Collembola, Thysanura) the young which hatches from an egg is a miniature of the adult and is called a nymph, it differs from the adult in having immature reproductive organs; by several moultings and growth it becomes an adult.

These insects are primitively wingless, they are also called Apterygota, e.g., Lepisma, the change from young to adult is negligible, such insects are ametabolic because there is no metamorphosis.

Type # 2. Heterometabolic Metamorphosis:

In winged insects the adult differs in several respects from the young, such insects are said to undergo metamorphosis in becoming adults. The nymph which hatches from the egg has a general resemblance to the adult in body form, type of mouth parts and possession of compound eyes, though these nymphs may have adaptations associated with their particular habits of being aquatic, swimming or burrowing.

In these the change from nymphs to adults is a gradual process in which appendages, mouth parts, antennae and legs of the nymph grow directly into those of the adult.

Wings develop gradually as external outgrowths of thorax and are visible externally in the nymphal instars, because of their external wing development they are also called exopterygota. The reproductive organs mature gradually. Insects showing this slight change from nymph to adult are known as heterometabolic (gradual), they include Dictyoptera, Orthoptera, Isoptera, Hemiptera and Anoplura.

Though nymphs of dragon flies, may flies, etc., are quite different from the adult in having special nymphal adaptations because their nymphs are aquatic, while the adults are aerial, the nymphal adaptations are shed in changing into adults, such forms with slightly greater changes are called hemimetabolic (incomplete), they include Odonata, Plecoptera and Ephemeroptera.

Type # 3. Holometabolic Metamorphosis:

In Lepidoptera, Coleoptera, Hymenoptera, Diptera, Siphonoptea, etc., the young which hatches from the egg is called a larva, the larva is very different from the adult in structure, body form, mouth parts, legs and in its mode of life, the larva has lateral ocelli in place of compound eyes, it feeds voraciously, grows, moves about and undergoes ecdyses.

The larva is so different from the adult that it first changes into a resting, quiscent instar called a pupa which is often enclosed in a cocoon secreted by the labial glands of the larva. Great transformation occurs in this instar, wings develop internally from pockets of the hypodermis, and they are not visible from outside.

Because wings develop from internal imaginal discs these insects are also called endopterygota. Appendages are formed, muscles, tracheae and parts of the alimentary canal are replaced by corresponding organs of the imago. Such vast changes are called holometabolic metamorphosis.

In holometabolic insects there is an internal reconstruction during late larval and pupal instars. Larval organs, with the exception of central nervous system and developing reproductive organs, are disrupted, their breaking down is called histolysis, this is brought about by phagocytes which feed on the organs, and products of their digestion are then used for building new structures.

The building of new structures is brought about by growth centres called imaginal buds or discs. Imaginal discs are groups of formative cells which are set aside in the larva, they are the rudiments of future organs of the imago, they form legs, mouth parts, internal organs and wings.

This process of formation of organs of an imago from imaginal discs inside the pupa is known as histogenesis and it results in the formation of the imago

Thus, two postembryonic processes occur in all insects, the first is growth in the young and the second is metamorphosis, in both of which moulting takes place; both processes are controlled by hormones of endocrine glands. Insects have two such endocrine glands, they are corpora allata and prothoracic glands.

The juvenile hormone of corpora allata controls growth and moulting up to the end of the larval period. So long as the juvenile hormone of corpora allata is produced the final moulting into a pupa or into an adult cannot take place.

The prothoracic glands are a pair of small glands in the first thoracic segment, they produce a hormone called ecdyson which brings about moulting and development of imaginal discs and reproductive organs.

When both hormones are secreted, then moulting of the larva only will take place. The result of the two hromones is supression of adult characters from appearing during larval and pupal instars. When only ecdyson is secreted, and the juvenile hormone is not produced, then the larva will moult into a pupa, and the pupa into an imago.

Thus, it is seen that ecdyson is essential for each moulting, but its action is modified as long as the juvenile hormone is present. Removal of the old cuticle in ecdysis is brought about by an enzyme secreted by the hypodermis, the enzyme erodes the lower surface of the cuticle, then the hypodermis secretes a new cuticle below the old one.

Metamorphosis: Kinds, Events and Role of Hormones this article we will discuss about Metamorphosis in Insects:- 1. Meaning of Metamorphosis 2. Types of Metamorphosis 3. Events 4. Role of Hormones.

Meaning of Metamorphosis:

Metamorphosis can be defined as “a rapid and complete transfor­mation from an immature larval life to a sexually adult form involving morphology, function and habitat changes”.

Ecdysis or moulting is the periodic shed- ding off the old exoskeleton. The duration of the period between two successive moults of a developing insect is called stadium. The form of the developing insect between two moults is called instar.

A larva is a motile, immature feeding stage in arthropods which is morphologi­cally different from the adult stage. The larvae of hemimetabolous insects are called nymphs. The adult stage of holometabolous insects is called imago.

Types of Metamorphosis:

On the basis of degree of changes there are 5 basic types of metamorphosis seen in insects.

They are:

(1) Ametabolous deve­lopment or Ametamorphic

() Gradual meta­morphosis or Paurometabolous develop­ment

(3) Incomplete metamorphosis or Hemimetabolous development

(4) Complete metamorphosis or Holometabolous devel­opment and

(5) Hypermetamorphosis or Hypermetabolous development.

(1) Ametabolous Developmentor Direct De­velopment:

Ametabolous type of development is called when the insects undergo little or no metamorphosis. Here the young’s emerge from the eggs resemble the adults in all respects except in size and sexual structures. It grows only in size by replacing its old skin through a process, called moulting.

The young which emerges from the egg resem­bles a miniature adult, called nymph. In nymph the reproductive organs are undevel­oped, and after several moults the nymph becomes an adult. This type of development is seen in apterygotan (wingless) insects (e.g., Lepisma, Fig. 18.134 and spring tails or Collembola, etc.).

(2) Gradual Metamorphosis or Paurometabolous Development:

This type of metamorphosis is seen in less primitive forms like cockroaches, grasshop­pers (Fig. 18.135), mantis and white ants, etc. Here the newly young which comes out of egg closely resembles the adult in general body form, habits and habitat but many adult features, i.e., wings and reproductive organs are undeveloped and their relative proportions of the body also differ. The young’s re called nymphs.

The wings de­velop as wing pads in the second and third thoracic segments at early age and gradually increase in size by mitosis in each moult. The external genitalia develop gradually at each moult. These nymphs lead an independent life and attain adult form through several moults.

This type of metamorphosis is called gradual metamorphosis or paurometabolous development because the young undergoes slow but steady change in each moult and attains the adult form.

Sometimes the gradual metamorphosis or paurometabolous devel­opment is included under hemimetabolous development. In each moult the proportion of the head gradually becomes smaller and the abdomen becomes longer.

In the life cycle of these insects there are three stages, e.g., egg → nymph → imago (adult). There is no pupal stage.

(3) Incomplete Metamorphosis or Hemimetabolous Development:

The insects which attain adult forms by gradual morphological change with succes­sive moults are called incomplete metamor­phosis or hemimetabolous development.

In many insects, e.g., dragonflies (Fig. 18.136), mayflies and damselflies, the different stages of the life cycle resemble to paurometabolous development except the nymphs are called naiads which are aquatic and respire by ex­ternal gills but the adults are terrestrial.

When these nymphs are ready to be adult they come out of water and adult winged forms are released. The wings and genitalia develop externally but are not fully formed until adult­hood. No further moulting takes place after the formation of wings, only exception in mayfly where winged form comes out of aquatic nymph and rests on a tree to undergo another moulting to become an adult.

In the naiads of hemimetabolous insects there are 3 pairs of thoracic legs, a head with compound eyes, antennae and small abdomen with posterior tracheal gills.

The naiads when attain adult stage after several moults, the head of the adult becomes pro­portionately smaller but the abdomen be­comes larger. The tracheal gills are lost and spiracles appear for aerial breathing. Many insects live for longer period as nymphs and the adult stage is short, the chief purpose of which is multiplication.

The best example is the may fly where adult stage lasts only for a day but nyphs take one year to grow.

From the egg → nymph → adult cycle of incomplete metamorphosis, it is evident that insects which are advanced from the primi­tive Lepisma like forms, have started to ex­plore two types of environments. But success in such attempts is achieved in forms with complete metamorphosis.

This climax of double life has been attained through a cycle of egg → larva → pupa adult; larva existing in an altogether different environ­ment than the adult. Nearly 87% of known insects develop through this cycle which involves two changes of form—one is from egg to caterpillar and the other from caterpillar to pupa and the adult.

(4) Complete Metamorphosisor Holometabolous Development:

Complete metamorphosis or holometabolous development is a kind of rapid mor­phological change during post embryonic transformation in some forms of insects where larva has no similarity with the adult and there is always a pupal stage. Complete metamorphosis takes place in beetles, caddis- flies, butterflies, moths, mosquitoes, flies, bees and wasps (Fig. 18.137).

In the house-fly (order Diptera) the larva is worm-like and devoid of appendages. It is called maggot. The mature larva is about 12 mm long. The head is indistinct and with a pair of oral lobes and hooks.

In case of beetle (Order Coleoptera) the larva is known as grub. The body of the grub is thick and with thoracic legs and well-developed head. They are usually sluggish in nature.

In the moths and butterflies (Order Lepidoptera), the larva is known as Caterpillar, which possesses a distinct head with power­ful mandibles and three pairs of jointed thoracic legs.

The abdomen possesses four or five pairs of un-jointed, short abdominal legs or also called pseudo-legs or prolegs. The caterpillars are often with protective colour or defensively shaped. These larvae eat voraciously and grow rapidly with sev­eral moultings. After sometime the larva is transformed into a stage, called pupa.

Again, these above mentioned larvae are also included under three categories, such as the maggot is called apodous larva for the absence of appendages on thorax and abdo­men and segmented body with a small head with sense organs. The larvae among beetles are also called campodeiform or oligopod larvae for the resemblance to apterous Campodea (Order Thysanura).

The larvae are characterised by without abdominal append­ages except cerci and the skin of the body is thick, provided with thoracic legs and sense organs. The caterpillar type larva is also called polypod or eruciform larvae (Fig. 18.138) which is characterised by a fleshy body with a thin skin and prolegs on the abdomen and six legs on the thorax.

The pupa is the third stage in the life of holometabolous insects and usually immo­bile and often remain within a protective covering from predators, called cocoon. In the mosquito the pupa is very active but does not eat anything. Though the pupal stage considers as a quiescent stage but it undergoes many internal changes. There are three types of pupa among the holometa­bolous insects.

They are:

(i) Exarate pupa:

This type of pupa is very common and found in almost all holometabolous insects (Fig. 18.139A) except Lepidoptera. The pupa is chara­cterized by the presence of free legs and appendages and the abdomen is capable of movement. This type of pupa is called free pupa.

(ii) Obtect pupa:

This type of pupa is seen among moths and butterflies and is characterized by the wings, and ap­pendages are not moved and fixed to the body by a moulting fluid. The pupa of butterflies is called chrysalis and it possesses a slender stalk at the top by which the pupa remains at­tached to the twigs.

(iii) Coarctate pupa:

This type of pupa is seen among dipterans (Fig. 18.139B). The pupa of house-fly is enclosed by a hard barrel-shaped chitinous case, called puparium. The puparium is segmented externally and the spiracles remain projected outwardly. The pupa of mosquitoes is comma-shaped and contains broad anterior cephalothorax.

The dorsal side of thorax bears a pair of small respiratory trumpets; the openings are guarded by numerous hairs. The abdomen is nine segmented and a pair of paddles on the eighth segment by which the pupa swims. After a period of pupal existence, the young insect emerges out by breaking or dissolving the pupal case. The holometabolous insects include 4 stages in their life cycle.

Egg → Larva → Pupa → Imago (adult):

Prepupa:

In the holometabolous insects, a stage is seen before the pupal stage, called prepupa. During this stage the feeding usu­ally stops and sometimes a cocoon is pro­duced. The prepupa resembles the larva but it is often shrunken and less pigmented.

(5) Hypermetamorphosis or Hypermetabolous Development:

It is a kind of metamorphosis in which there are two or three distinct types of larval instars with different habits and structures found in certain insects. This type of meta­morphosis is seen in blister beetles (Fig. 18.140).

Events of Metamorphosis:

In recent years considerable amount of work has been done to understand the events of metamorphosis. It has been found that at the time of early development, in the deve­loping egg the cells are segregated into two groups—one group for working at the larval life and the second group to take charge during pupal and adult life.

In a growing larva, the larval cells increase only in size but never undergo division. The second group of cells, called imaginal buds and discs, re­main inactive in the body of larva. When the larva is full-grown, second group of cells take over the charge. Within the apparently inactive pupa tremendous activities go on at cellular level. Imaginal buds grow by divi­sion.

During metamorphosis, most of the larval organs in the pupa except the central nervous system and developing reproduc­tive organs are broken down by enzymes and the process of disintegration of the lar­val organs is called histolysis and these larval disintegrated cells die and is used up by the imaginal cells.

In certain insects, within larva, pupal cells become fluid in consistency and imaginal cells continue to form the adult structures. The imaginal buds are the groups of formative cells but remain inactive in the larva but form the rudiments of future organs by mitosis.

These formative cells set aside in the pupa and reach func­tional organs by differentiation in the imago (adult). The process of formation of tissues and organs from the imaginal buds, called histogenesis. The wings, mouth parts, inter­nal organs, muscles and legs develop from the imaginal buds.

Role of Hormones during Metamor­phosis:

It has also been well established that the moulting and metamorphosis in insects are controlled by hormones (Fig. 18.141). The secretions of three organs are related to this process.

These organs are:

(i) The brain (protocerebrum)

(ii) The prothoracic gland and

(iii) The corpora allata.

(i) The Brain (protocerebrum):

In the brain there are four groups of neurosecre­tory cells. Of these two groups lie on the midline and another two group’s lie on the sides-one group on each side. The neurosecretory cells secrete a kind of protein hor­mone, called prothoracotropic (PTTH) or brain hormone that activates the prothoracic glands which in turn produce moulting hormone.

The protocerebrum sends the neuro­secretory axons to the corpora cardiaca, a pair of small glands which lie posterior to the brain. Prothoracotropic hormone passes to the corpora cardiaca along the axons where it is released to the blood.

(ii) Prothoracic Glands (PG):

There are a pair of glands located in the prothoracic region. These are also called moult gland or ecdysial gland. Each gland appears V-shaped and is a mass of glandular tissue of non- nervous origin.

It produces a hormone, known as ecdysone. It is steroid in nature. Butenandt and Karlson (1951) first isolated from the pupa of silkworm. Its empirical formula is C27H44O6. The hormone stimu­lates growth and initiates the process of moulting and shedding the old cuticle of the larva and the new cuticle is formed beneath the old cuticle.

(iii) Corpora Allata:

They are paired non- nervous secretory cells and situated behind the brain posterior to corpora cardiaca. The corpora allata secrete another fat soluble hormone, called juvenile hormone (JH). Chemically it is related to ecdysone and also a steroid in nature. The juvenile hormone keeps the larval cells active and also controls the qualitative changes in the body during metamorphosis.

As long as the juvenile hor­mone is secreted from the corpora allata, the pupa and imago (adult) stages are not devel­oped. After a certain period the production of ecdysone instructs to stop the flow of juvenile hormone on one hand and on the other hand triggers the imaginal buds to be active.

The absence of juvenile hormone causes the death of larval cells and they are used as nutrients for the growing imaginal buds. If the amount of juvenile hormone (JH) becomes lower in the blood, the moulting from larva to pupa takes place and absence of juvenile hormone in the blood, there oc­curs from pupa to adult moult. So it has been determined that the process of moulting is under hormonal control.

Metamorphosis in Amphibia

Metamorphosis of Amphibians| Phylum Chordata

In this article we will discuss about Metamorphosis of Amphibians:- 1. Meaning of Metamorphosis 2. Types of Amphibian Metamorphosis 3. Structure 4. Hormonal Control.

Meaning of Metamorphosis:

Metamorphosis may be defined as “a rapid differentiation of adult characters after a rela­tively prolonged period of slow or arrested differentiation in a larva”.  

According to Duellman and Trueb (1986) Metamorphosis can be defined as “a radical transformation from larval life to the adult stage involving structural, physiological, bio­chemical and behavioural changes”.

Types of Amphibian Metamorphosis:

1. Progressive metamorphosis:

During metamorphosis if the animal pro­gresses in the evolutionary grades, the meta­morphosis is considered as a progressive meta­morphosis; e.g., in most anurans of Amphibia.

2. Retrogressive metamorphosis:

When metamorphosis takes place in lower direction, i.e., by metamorphosis the animal retrogresses or shows indication of degeneration in the scale of evolution, called retrogressive metamorphosis; e.g., Ascidia of urochordates or in neotenic forms like salamanders.

Metamorphic changes of amphibians:

Etkin (1968) have divided three stages:

a. Premetamorphic stage:

The stage is characterized by the consi­derable growth and development of larval structures but metamorphosis does not occur.

b. Prometamorphosis:

The stage is characterised by the conti­nuous growth specially the development of limbs and initiation of metamorphic changes.

c. Metamorphic climax:

The stage is characterised by the radical changes in the features of the larva, and climax is considered by the loss of most larval features.

Structure of Metamorphosis:

Structure of a freshly hatched tadpole larva:

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1. A freshly hatched tadpole larva has a limbless body.

2. The body is divided into an ovoid head, a short trunk and a slender tail.

3. A small opening situated ventrally at the root of the tail is known as anus.

4. An adhesive sucker is present on the ven­tral side of the head by which the tadpole larva attaches itself to the aquatic weeds.

5. The mouth is lacking and as a result it cannot take anything from outside.

6. The yolk material provides the nutrition.

7. The respiratory organs comprise of three pairs of highly vascular and branched feathery external gills.

8. After a few days the mouth is formed near the sucker.

9. A pair of horny jaws surrounds the mouth.

10. The tail becomes more elongated and develops a dorsal and a ventral fin.

11. V-shaped myotomes develop on both the sides of the tail.

12. At this time this free-swimming tadpole larva ingests aquatic weeds, as a result of which the alimentary canal becomes extremely elongated.

13. To accommodate such a long alimentary canal inside the cavity of the short trunk, it becomes spirally coiled like the spring of a watch.

Structure of an advanced tadpole larva:

1. In the advanced stage, the pharynx of the tadpole larva becomes perforated by gill- slits.

2. External gills disappear and the internal gills are formed between the gill slits.

3. The gills and the gill-slits are covered by the operculum (or gill-cover).

4. Thus the tadpole larva has three pairs of external gills at the start which are subse­quently replaced by three pairs of internal gills.

5. In the larval stages, the arterial arches also show modifications in terms of both exter­nal and internal gills (Fig. 7.28).

6. The operculum fuses with the trunk on all sides except a small opening, called spiracle on the left side.

7. Water enters into the pharynx through the mouth and goes out through the spiracle.

8. During this transit of water the internal gills are bathed with water containing oxygen dissolved in it.

9. While the internal gills are functioning, a pair of lungs develops as outgrowths from the pharynx on the ventral surface.

10. The hind limbs appear prior to the fore­limbs.

11. The forelimbs remain first hidden under the operculum and subsequently emerge through it.

12. At this stage both the internal gills as well as the newly formed lungs are functional.

13. When the lungs become fully developed, the internal gills become degenerated.

14. At this stage it looks like a miniature toad except having a tail.

15. As the limbs are developing, the animal enters into a period of starvation.

16. The material of the tail becomes eventu­ally absorbed into the body.

Structure of a freshly formed toad:

1. After the absorption of tail, the young toad leaves the primal aquatic home and comes to the land and hops.

2. The mouth becomes wider and a pair of true bony jaws replaces the horny jaws.

3. It now changes its food habit to become carnivorous type, as a result the alimen­tary canal becomes short and less coiled.

The changes that take place in the tadpole can be divided into four groups.

They are:

1. Changes of tadpole in habit and habi­tat:

(i) With the metamorphosis, the meta­morphosed larva leaves aquatic medi­um and frequently visits the land.

(ii) The herbivorous tadpole larva chan­ges into carnivorous specially con­sume the insects (insectivorous).

(iii) The praying habits develop by the adults and the adult animals become more active and swift moving.

(iv) In the first stage of adult toad, they jump into nearby pond and in other aquatic medium, and then jump on the land by their elongated hind limbs.

2. Morphological metamorphic changes:

a. Regressive changes:

(i) The tissues of tail and tailfin are com­pletely absorbed into the body.

(ii) The horny jaws with teeth are shed and mouth becomes a large transverse slit.

(iii) The external gills disappear and the gill slits communicate to the pharyn­geal cavity.

(iv) The length of the alimentary canal much reduces.

(v) The changes of the blood vascular system take place and ultimately some blood vessels are reduced.

(vi) The lateral line sense organ disap­pears.

(vii) Operculum and spiracle disappear.

b. Progressive changes:

(i) The fore and hind limbs increase in size.

(ii) The tongue becomes long and more elastic which is free and bifid posteri­orly.

(iii) The eyes become large and prominent and develop eye-lids and nictitating membrane.

(iv) External nostrils communicate with buccal cavity through internal nostrils.

(v) Tympanum and middle ear develop.

(vi) Liver becomes more enlarged.

(vii) Three chambered heart develops from two-chambered heart.

(viii) Pronephros is replaced by mesonephros.

3. Biochemical changes during meta­morphosis:

(i) The concentration of serum protein becomes about double during meta­morphosis.

(ii) Biosynthesis and concentration of haemoglobin are greater in adult than in larvae.

(iii) In the liver, DNA synthesis, lipid syn­thesis, enzymes for ornithine urea cycle increase during adult stage.

iv) Alkaline phosphatase and hydrolase decrease in adult stage of the anurans.

4. Changes in Physiology:

(i) At the beginning of metamorphosis, the pancreas starts to secret insulin and glucagon hormones. This is rela­ted to the increased role of the liver.

(ii) During the larval stage, the end pro­duct of nitrogen metabolism is ammo­nia. But after metamorphosis, the toads and frogs excrete most of their nitrogen in the form of urea. This is a shift from ammonotelism to ureotelism with the change of environment from aquatic medium to land.

Hormonal Control for Metamorphosis:

Two hormones such as Triiodothyronine (T₃) and Tetraiodothyronine (T₄) or thyroxine are necessary for biochemical and morpholo­gical changes during anuran metamorphosis. These thyroid hormones are produced by the induction of anterior pituitary lobe or pars distalis when it reaches certain degree of differentiation.

Then it is capable to synthesize a hormone, thyrotropin (Thyroid Stimulating Hormone, TSH) which acts on the thyroid, stimulating the production and secretion of triiodothyronine (T₃) and thyroxine.

In pre-metamorphic stage the prolactin level is high but levels of thyroid stimulating hormone (TSH) and thyroid hormone (T₃, T₄) are low. The hypothalamus – pituitary link is poorly developed. In pro-metamorphosis, the hypothalamus and pituitary link develops.

The prolactin level is low but the levels of thyroid stimulating hormone (TSH) and thyroid hormones (T₃, T₄) are high. In metamorphic climax, the pro­lactin level increases suddenly, then maintains steady low level. The TSH is high until end of climax and the thyroid hormone (T₄) level becomes low.

Metamorphosis of Toad:

The young tadpole larva resembles a fish. It leads an independent and self-supporting life. This fish like tadpole larva completely metamorphoses into toad, is exclu­sively a progressive process. According to Mohanty-Hejmadi and Dutta (1978) – deve­lopment is rapid being completed in 34-52 days. Daniel (1963) reports the hatching in about 4 days after laying.

Short 2 mark Question and its answer Developmental biology

1)Von bears postulates

Von Baer’s principles of embryonic differentiation constitute a
better guide to embryological evidence for evolution. These
principles are as follows:
1) General characteristics appear in the development early and
specialized characters latter on.
2) From the more general, the less general and finally the specialized
characters appear.
3) An animal during development departs progressively from the
form of other animals
4) Young stages of an animal do not resemble with their embryos

2)Teratology?
Teratology is the study of abnormalities of physiological development. It is often thought of as the study of human congenital abnormalities, but it is broader than that, taking into account other non-birth developmental stages, including puberty; and other organisms, including plants.

3)Stratification 
is a process of treating seeds to simulate natural conditions that the seeds must experience before germination can occur. Many seed species have an embryonic dormancy phase, and generally will not sprout until this dormancy is broken.
4)Teratogen: Any agent that can disturb the development of an embryo or fetus. Teratogens may cause a birth defect in the child. Or a teratogen may halt the pregnancy outright. The classes of teratogens include radiation, maternal infections, chemicals, and drugs.
5)Acrosome:
The acrosome is a lysosomal-like compartment derived from the Golgi. It has a low pH and contains
soluble hydrolases (serine protease acrosin). In cross-section through the head of a sperm, one would
cross four membranes in traversing from the plasma membranes to the nuclear membrane. During the
acrosome reaction, fusion of the outer acrosomal membrane with the plasma membrane releases the
contents of the acrosome and exposes the inner acrosomal membrane as the functional outer boundary of
the sperm head