Development in frog
(i) Breeding
- Frog breeds in the rainy season, June to September.
- Male frogs produce crocking sound (mating call) by their vocal sacs.
- The sexual embrace of the male and female frogs is called am plexus (false copulation).
(ii) Ovulation
- Ovulation is the release of eggs from ovary in the body cavity.
- The eggs in the stage of secondary oocytes are released into the body cavity by rupture of ovary during ovulation.
(iii) Spawning
- Spawning is the act of laying of eggs by the female frog stimulated by the male during am plexus.
- Spawn is a cluster or mass of eggs laid by a female.
- a spawn of Rana Tigrinya contains about 3000-4000 eggs.
- The diameter of frog’s egg varies from about 0.75 to 2.5 mm.
- The egg is surrounded by a thin vitelline membrane and three layers of jelly coats made of gelatin.
- Gelatin protects the egg from predators and also acts as an insulator keeping the egg warm.
(iv) Fertilization
- Fertilization in frog is external taking place in water.
- The sperms are released on the egg mass before it reaches water.
- When a sperm enters into the egg of frog, second meiotic division occurs.
- A sperm enters into the ovum at some point in animal hemisphere.
- A gray crescent appears in the equatorial zone geometrically opposite to the sperm entrance.
- Gray crescent marks the dorsal side of the future embryo.
- Sperm entrance point marks the anterior side of the future embryo.
- The bilateral organization is established at the time of sperm penetration.
- The region where sperm enter the egg cell is called ‘reception cone’.
- Entry of sperm induces following changes in the ovum
q Formation of fertilization membrane around the ovum.
q Completion of second maturation division of egg nucleus.
q Activation of the egg for development.
q Fusion of male and female pronuclear, i.e., amphimixis.
q Formation of gray crescent.
(v) Structure of egg
- Frog’s egg is mesolecithal (based on distribution of yolk).
- Upper black of darkly pigmented part is animal hemisphere. Lower pigmented or white part is vegetal hemisphere.
- Cytoplasm is concentrated in animal pole. It is directed dorsally and pigmented animal pole is related with camouflage, to escape notice of predators.
- Vegetal hemisphere of frog’s egg contain yolk. It remains directed downwards.
- The correct sequence in the development of frog is fertilization, cleavage, morula, blastula and gastrula.
(vi) Cleavage
- Cleavage is a term used for the early cell divisions of the zygote upto the completion of blastula stage.
- First cleavage of frog is meridional passing through median longitudinal axis, holoblastic and equal.
- The first cleavage furrow appears at animal pole and results in two blastomeres, right and left.
- The second cleavage is right angle to first one, again meridional results in four blastomeres, identical with respect to cytoplasm, pigment and yolk gradient.
- Second cleavage is again homoplastic and equal.
- Gray crescent is present only in two blast meres of future dorsal side.
- All divisions from third cleavage are unequal holoblastic.
- Holoblastic equal cleavage in frog ends after second cleavage division.
- Third cleavage plane is horizontal, but above the level of equator (latitudinal).
- Third cleavage results in the formation of embryo with eight blastomeres, four upper one are smaller and pigmented and four lower most are larger and yolk laden.
- Smaller cells are micromeres and larger ones are called macromeres. The micromeres contain no yolk and macromeres contain large amount of yolk.
- Fourth cleavage involves two synchronous meridional divisions resulting in the formation of 16 blastomeres.
- Fifth cleavage involves two simultaneous latitudinal divisions resulting in the formation of 32 blastomeres.
- The divisions after the fifth cleavage becomes very irregular and asynchronous. The micromeres divide faster than macromeres.
- After sixth or seventh cleavage division, the embryo looks like a mulberry-shaped ball of cells. This called morula stage.
- Modula is a solid ball of cells. A cavity called blastocoel appears towards animal hemisphere. The blastula of frog is called coeloblastula.
- Blastocoel is filled with an albuminous fluid secreted by surrounding cells.
- The two cells of frog’s egg formed by the first cleavage represent the right and left half of the embryo.
- In frog, there is a regulative development. The cleavage is indeterminate. If one of the two blastomeres of frog is damaged, the development will be normal.
(vii) Gastrulation
- Gastrulaion is the process by which a blastula is converted into gastrula.
- Blastula is a hollow ball of cells. By the end of gastrulation, it is converted into a three-layered embryo made of ectoderm, mesoderm and endoderm often enclosing an archenteron.
- Gastrulation includes three kinds of morphogenetic movements of cells namely
q Epiboly of ectoderm
q Invagination of endoderm
q Involution of chorda mesoderm
- Epiboly is migration and spreading of micromeres over the embryo is known as epiboly.
- Invagination of prospective endoderm cells occurs below equator, exactly below the midpoint of gray crescent of blastula. It results in the formation of a slit later giving rise to blastopore.
- Involution is the in sinking and movement of chorda mesoderm cells towards anterior side along the roof of blastocoel.
- Gastrulation results in the formation of a new cavity, archenteron which opens outside through blastopore.
- Archenteron is present the lumen of future gut.
- Blastopore occurs in gastrula and opens into archenteron.
- Blastopore will give rise to future anus in frog.
- If involution does not takes place (chorda mesoderm cells vaginitis instead of involution) no gastrulation takes place. The development will be stopped.
- Ingression is the migration of individual vegetal cells to the interior of the embryo.
- By the end of gastrulation, blastocoel will be reduced. A yolk plug of endodermal origin closes the blastopore.
- Posterior side of future tadpole is represented by the side of frog’s embryo bearing the yolk plug.
(viii) Organizer
- The dorsal lip of blastopore in the amphibian gastrula is called primary organizer.
- The theory of organizer (inductor) in amphibian was introduced by Spemann in 1938. He was awarded Nobel prize for this work.
(ix) Neurulation
- Neurulation takes place after gastrulation. During this stage a neural tube is formed.
- The embryo lengthens along its anterior-posterior axis, neural plate (ectodermal) become thickened and raised above the general level as ridges called neural folds.
- Neural folds meet and fuse at the mid dorsal line.
- Neurulation includes the formation of neural tube, notochord and gut.
- Formation of notochord is known as notogenesis.
(a) Post neurular development
q The development takes place inside egg membrane upto tail bud larval stage.
q Hatching occurs in 6th day of embryonic life.
q During hatching, the young frog is called tadpole larva.
q Newly hatched tadpole larva remain attached to aquatic plants by its oral sucker.
q After 24 hours of hatching, mouth and anus are perforated.
q The larval body is elongated forming head, trunk and tail.
(b) External gill stage of tadpole
q Just above one day after hatching, the external gill stage starts.
q Eyes become fully developed and functional.
q Horny jaws with teeth appear along the rim of mouth.
q Tail elongates and becomes a powerful swimming organ.
q Pronephric kidneys become fully developed. Frog’s tadpoles are ammonotelic. Nitrogenous waste matter excreted by frog’s tadpole is ammonia.
q The branchial clefts are perforated and finger-like external gills project from the sides of the head in branchial region.
q External gills are three pairs in number; tadpole respires mainly by gills using oxygen dissolved in water.
q Gut is differentiated into pharynx, oesophagus, lung rudiments, stomach, liver, gall bladder and intestine.
q Tadpole is herbivorous (phytophagus), feeds on aquatic plants.
q Tadpole has a long coiled intestine because digestion takes place relatively long time.
(c) Internal gill stage of tadpole
q Tadpoles grow older, the hind limb buds and internal gills develop.
q External gills are replaced by four pairs of internal gills covered with a fold of skin called operculum.
q Operculum encloses a chamber, opercular chamber, opens to exterior by spiracle.
q Spiracle is present only on the left side of the tadpole.
q During respiration, the water currents enter the mouth, bath the gills of pharynx and exit through the spiracle.
q Oral sucker disappear and lateral line receptor develop. This serves to perceive stimuli of movements, currents and vibrations of water.
q A tadpole of frog resembles a fish in many features except that the tadpole does not possess paired fins and scales on the body.
q The tadpole cannot survive when exposed to air because its skin is thin and delicate.
q Exposure of tadpole to land leads to dehydration and death.
q First sign of metamorphosis is the appearance of hind limbs.
q End of tadpole in the life history is marked by appearance of forelimbs.
(x) Metamorphosis
- Metamorphosis is the abrupt transition from larval to adult form.
- Metamorphosis includes morphological, anatomical, physiological and behavioural changes.
- Two or three weeks after breathing with gills, the tadpole larva undergoes drastic changes called metamorphosis.
- Two types of changes during metamorphosis are regressive and progressive.
- Some of the regressive morphological changes are:
Disappearance of larval sucker, long tail, gill clefts, internal gills and lateral line sense organ.
- Some of the progressive morphological changes are:
q Formation of forlimbs breaking through operculum.
q Replacement of pronephros with mesonephric kidney.
q Enlargement and development of hind limbs.
q Development of tongue and teeth.
q Thickening of skin and development of mucous glands.
q Two chambered heart becomes three chambered.
q Development of lungs.
- During metamorphosis, the disappearance of larval organs is by histolysis and formation of adult organs is by histogenesis.
- The tail is shortened by reabsorption with the help of lysosome enzyme cathepsin. This process is also known as
autolysis.
- Nervous system undergoes no special changes (least changes) during metamorphosis.
- Respiratory system undergoes maximum changes during metamorphosis.
- During metamorphosis, skin gets cornified and mucous and poison glands develop.
- The feeding habit changes from herbivorous habit of tadpole to carnivorous habit of adult.
(xi) Hormonal control of metamorphosis
- Hormonal control of metamorphosis in amphibian was discovered by Gudernatsch (1912).
- Metamosphosis occurs only when adequate amount of thyroxine is secreted by thyroid of tadpole.
- Since iodine is the main constituent of thyroxine, it is found that deficiency or abundance of iodine in pond water also affect metamorphosis.
- When there is no iodine in pond water or thyroxine is not secreted in the body, metamorphosis fail to begin. The tadpole continues growing and becomes abnormally large.
- If thyroxine is given, the tadpoles metamorphose too rapidly giving rise to vary small black frogs which soon die.
- If thyroids are removed, giant tadpoles can be reared which are unable to metamorphose.
- On removing the thyroid from the tadpole of frog, it will remain tadpole throughout life.
- If a small quantity of thyroid extract is added to water in which frog tadpoles are present, it will hasten the metamorphosis.
- The endocrine gland which initiates metamorphosis in frog is thyroid.
- Thiourea is antithyroid drug, it inhibits metamorphosis of frog.
- Neoteny refers to the retention of a larval or embryonic trait in the adult body, e.g., Cartilaginous skeleton in adult
chondrichthyes and larval gills in some adult salamanders.
- Paedogenesis or paedomorphosis refers to development of gonads and production of young ones by larval or pre adult animal e.g. liverfluke and ambystoma.
- Deficiency of iodine in the soil results in the failure of metamorphosis in ambystoma.
- Axolotl is the larva of ambystoma, it shows paedogenesis.
Important Tips
- Metamorphosis: Transformation of young into a morphologically and physiologically different adult is called metamorphosis.
- Retrogressive metamorphosis: When an advanced larva changes into a degenerate adult e.g. Herd mania, Sacculina.
- Progressive metamorphosis: When a simplified larva changes into an advanced adult e.g. Frog.
- Embryonic induction: Morphogenetic effect of one embryonic part on another embryonic part. Inducing embryonic part is called inductor or organizer, while responding embryonic part is called responding tissue. This induction is through certain chemicals called evocators.
- Primary organizers include dorsal lip of blastopore; grey crescent (neural inductor) and chorda-mesoderm (induces forebrain).
- Secondary organizerg. optic area induces formation of lens.
- Tertiary organizerg. Lens induces formation of cornea.
