The development of frog is divided into two phases:
1. Pre-natal or Embryonic development
· It is the development that takes place inside the egg or inside the womb of the mother up to the gastrula stage from the zygote.

2. Post-natal or Organogenesis
· In this period, development from the birth of an organism to death is studied or the formation of organs from three germinal layers.

a. Breeding:
· Breeding takes place during the rainy season, in water.
· Male frog croaks (mating call) by staying at shallow water region at night time to attract females. 
· Many female frogs attract towards a male frog and male frog mounts over a female frog.
· This Sexual embracing condition is called amplexus or pseudocopulation.
· Then the females lay the eggs in water and the male also releases the sperms.

b. Ovulation:
· It is a process of release of secondary oocytes from the ovary to the body cavity.

c. Spawning:
· When the eggs are laid by female frog stimulated by amplexus, the gelatin coat acquired around the egg in glandular oviduct swells to form a coherent mass called frog spawn.
· A spawn contains about 3000-4000 eggs.
· The diameter of eggs ranges from 0.75 -2.5mm.

Functions of Gelation:
· Protection from bacteria, spores and dust.
· Protection from predators
· insulation
· Check harmful radiations
· Provide sufficient aeration

d. Fertilization:
· Eggs of frog are Mesolecithal and Telolecithal.
· Eggs are surrounded by a vitelline membrane (secreted by cell membrane) and 3 concentric layers of gelatin or albumen (acquired in glandular oviduct).
· Upper, darkly pigmented animal hemisphere contains cytoplasm whereas the lower unpigmented vegetal hemisphere contains yolk.
· Black colour of the egg is due to the presence of melanin in albumen which protects the egg from UV radiation.
· Fertilization is external.
· During copulation, spermatic fluid is released near the eggs as soon as they are laid. Then sperms swarm towards the ovum with the help of the tail.
· Sperms are attracted by fertilizin (glycoprotein) emitted from the surface of the egg and antifertilizin (acidic protein) secreted from the cytoplasm of sperm.
It is called fertilizin and antifertilizin reaction.
· Sperms penetrate the eggs before the albuminous coat swells and binds the eggs into frogspawn.
· Sperms penetrate through the cone of reception in the animal hemisphere by secreting proteolytic enzyme lysine through its acrosome (modified Golgi bodies).
· When the sperm enters the ovum, second meiotic division occurs in the egg.
· Dissolution of the nuclear envelope of male and female pronuclei is followed by an arrangement of the chromosomes at the equator of the spindle.
· There is NO formation of nuclear envelope around the chromosomes. [amphimixis occurs]
·  Sperm entrance point marks the anterior side of the future embryo.
·  Changes due to fertilization:
·  Vitelline membrane becomes thick and impermeable, this ensures monospermy. Now this membrane is called the fertilization membrane.
·  Second meiosis of egg completes and second polar body is formed.
·  Diploid number of chromosomes is restored.
·  Grey crescent is formed opposite to the point of sperm entry which indicates the position of the future alimentary canal.
·  Cleavage or segmentation is initiated

e. Cleavage:
·  It begins 2.5 – 3 hours after fertilization.
·  In frog, cleavage is Holoblastic and unequal
·  After fertilization, 3 areas can be seen in the zygote:
a) UPPER ANIMAL POLE: Pigmented contains albumen.
b) LOWER VEGETAL POLE: Non-pigmented and contains yolk.
c) GREY CRESCENT: developed at opposite to the point of sperm entry.

· First cleavage of frogs is meridional, holoblastic and equal. So two blastomere are formed.
· Second cleavage is vertical which is laid down at right angles to the first one. So four blastomeres are formed.
· Third cleavage is horizontal but NOT equatorial (latitudinal) which lies more towards the animal pole.
· As a result, of which 8 daughter cells are formed: 4 upper smaller ones called Micromeres or Epiblasts & 4 lower larger ones called Megameres or Hypoblasts which contain a large amount of yolk.
· Fourth cleavage involves two synchronous meridional divisions so 16 daughter cells are formed (8 micromere at animal pole and 8 megameres at the vegetal pole).
· Fifth cleavages involve two simultaneous horizontal or latitudinal divisions forming 32 cells (16 micromeres and 16 megameres).
· Division after the fifth cleavage is very irregular and asynchronous.
· Micormeres divide faster than megameres.

f. Morula:
· After the sixth division, the embryo looks like a mulberry shaped solid ball of cells. This stage is called the morula stage. [Yolkless micromeres divide rapidly, yolk laden megameres divide slowly].
· In frogs, there is regulative development and indeterminate also.

g. Blastula:
· As segmentation proceeds, a fluid-filled space appears due to imperfect opposition of the daughter cells in the animal pole called incipient blastocoel or segmentation cavity.
· This cavity gradually enlarges and is known as Blastocoel and this stage is known as Blastula.
· Blastula of frog is thus Coeloblastula.
· The dorsal and lateral sides of the blastocoel is occupied by micromeres and the floor has megameres.
· At this stage, Presumptive areas begin to develop.
· Animal pole of embryo represents presumptive ectoderm which further divides into the presumptive epidermis and presumptive neural plate.
· A small portion near the vegetal half close to the animal half represents presumptive notochord and mesoderm.
· All these presumptive areas are formed from micromeres.
· Remainder of vegetal half represents the presumptive endoderm that develops from megameres.

h. Gastrulation:
· This stage involves, rearrangement of potentially differentiated presumptive areas, their movement. So that they come to occupy their definite position and proportion in embryo.
· Process of Gastrulation involves 3 processes:
    1. Epiboly or Overgrowth
    2. Emboly or Invagination
    3. Contraction of the lips of the blastopore

· Micromeres divide more rapidly and migrate towards the vegetal pole covering the presumptive notochord, mesoderm and endoderm except the yolk plug.

· In this stage, cells present behind the notochord begin to invaginate and migrate inward at the pigmented border of the grey crescent forming a crescentic depression that marks the beginning of archenteron.
· Archenteron lies just above the megameres and its exterior opening is called blastopore which represents the future anus of the frog. So, forg is deuterostome.
· As the archenteron increases in size, blastocoel becomes reduced.

· The dorsal, lateral and ventral lips of blastopore come closer to each other and form the complete circle or blastopore
· Some amount of endodermal tissue or megameres protrudes through the mouth of blastopore as yolk plug and this stage is called yolk plug stage.
· Archenteren (future gut) is formed which opens to the exterior via blastopore (future anus).
· Gastrulation results in the formation of an embryo with two germinal layers.
· Ectoderm is the externally covering the embryo and inner layer Chordamesoderm and endoderm are surrounding the archenteron

Stages of gastrulation in frog early gastrula, mid gastrula and completed gastrula

i. Involution:
· This is the insinking and movement of chordamesoderm cells towards the anterior side along the roof of the blastocoel and lie in between the ectoderm and endoderm of the embryo.
· Therefore roof of the archenteron is formed by notochord cells and sides of mesoderm cells.
· Ingression is the migration of individual vegetal cells to the interior of the embryo.
· At the end of gastrulation, the blastocoel is reduced and finally disappear due to the increased size of the archenteron.
· Posterior side of the future tadpole is represented by the side of the frog's embryo bearing the yolk plug.
· Dorsal lip of the blastopore of the gastrula is the primary organizer.

· After the completion of the gastrula, three germinal layers are formed which are as follows:
1. Ectoderms – It is formed by the epidermis and neural plate.
2. Chordamesoderm - It is formed by chorda cells and mesoderm.
3. Endoderm - It is formed by megameres of vegetal pole.

Postembryonic development or Organogenesis or Organogeny
A. Neurulation or Neural tube formation:

· It is formed by a presumptive neural plate of the gastrula.
· It becomes thickened forming a mid-dorsal pear-shaped medullary or neural plate.
· A pair of longitudinal ridges called Neural folds appear along the edges or lateral sides of the neural plate.
· So a middle neural groove is formed.
· These folds increase in height and fuse along the midline to form the Neural tube enclosing a neural canal.
· Its internal cavity is called neurocoel.
· Anteriorly, the neural tube remains open for a short time through a Neuropore and posteriorly, it communicates with archenteron through the Neuroenteric canal.
· Embryo lengthens and becomes oval. This stage is called Neurula.
· Anterior enlarged portion of the neural tube represents the future brain and the posterior tubular portion represents a future spinal cord.
· Certain mesodermal cells at the lateral margins of neural plate do not become incorporated in the neural tube.
· They remain separated as linear bands forming the neural crests. Later they give rise to the autonomic nervous system, dorsal ganglia of cranial and spinal nerves, trabeculae of skull and elements of visceral arches.

Formation of the neural tube (neurulation)

B. Notogenesis or Notochord formation:
· Chorda cells lying at the mid-dorsal region of archenteron separate from adjacent mesodermal cells. Then they form a long median rod-like thickening just below the neural plate. This is known as Notochord which has characteristics of vacuolated cells which form the notochordal sheath around the notochord.
· Notochord forms the axial skeleton.

C. Coelom Formation:
· The lateral plates (i.e. mesoderm) divide into three blocks – epimere, mesomere and hypomere.
· Epimere further divides into three layers- dermatome (gives dermis of skin), myotomes (gives muscles) and sclerotome (gives the axial skeleton).
· The mesomere later develops into nephrostomes and finally changes into nephrostomes or excretory and genital organs.
· The hypomere is responsible for the formation of the coelom in frogs.
· At first a split occurs in the middle of the hypomere which divides the hypomere into two layers:
    a. Outer layer: Somatic / Parietal layer
    b. Inner layer: Splanchnic / Visceral layer
· This small cavity inside these two layers is called splanchnocoel.
· As the size of splanchnocoel increases, the somatic layer pushes toward the body wall and is finally attached to it.
· Similarly visceral layer is attached to the gut wall forming a large cavity in between the body wall and the gut wall called the coelom or body cavity.
· Coelom of frog is schizocoel.

Fate of Primary Germ Layers

Epidermis & cutaneous glandsNotochord (axial skeleton) Epithelium of digestive tract
Lining of proctadeum & stomodeumPericardium, pleura, peritoneumGastric & intestinal glands
Epithelium of conjunctiva, auditory & olfactory organsAll muscles except the iris & ciliary bodyLiver, pancreas & their ducts
Lens, cornea, retinaSkeleton (bone + cartilage)Pharynx, tonsils, Eustachian tube, thymus, thyroid, parathyroid
Central nervous systemAll connective tissues including bloodRespiratory tract, larynx, trachea, lungs
Enamel of teethDermis of skinThe lining of the urinary bladder
Iris & ciliary bodyCardiovascular, excretory & genital systems
Adrenal medullaNeuroglia
Adrenal cortex
Eye parts except for lens, retina and cornea

D. Hatching:
· It Occurs at 2 weeks after fertilization.
· Newly hatched tadpole is Limbless with a head, fusiform body and short tail. It measures about 7 mm in length. The body surface is ciliated.
· Rudimentary eyes, nostrils and stomodeum are seen soon after hatching.
· Behind the mouth, Mucus/cement gland is present which help to attach to the aquatic weeds.
· On either side of the head, 3 pairs of feathery projections appear which are External gills.
· After about a week, the mouth appears, bordered by a pair of horny jaws bearing papillae / horny teeth for feeding.
· Alimentary canal rapidly lengthens to digest vegetable diet.
· Liver, pancreas, lungs, bladder appear.
· Fold of skin covers the gill slits (4 pairs) known as Operculum which bears a small opercular spout at the left.
· Tail is elongated with dorsal and ventral fins.

E. Metamorphosis:
· It is a series of changes that transform a larva into an adult.
· It is stimulated by thyroxine which contains iodine, delayed by storage of food.
· But thiourea is an antithyroid drug that inhibits metamorphosis.
· Metamorphosis is accelerated by Iodine whereas delayed by thiourea.

F. Behavioural changes
· Herbivorous to the carnivorous mode of feeding.

G. Structural changes
· Limb buds appear, lengthen and become differentiated.
· Tail is shortened by reabsorption with the help of the lysosomal enzyme cathepsin.
· This process is also known as autolysis (by wandering phagocytes).
· The fasting tadpole gets nutrition from degenerating tail.
· Horny jaws and teeth disappear, vomerine teeth, tongue and hyoid apparatus are formed.
· Gills disappear, lungs mature.
· Eyes are exposed, middle ear and tympanum formed.
· Heart becomes 3 chambered
· Skin becomes thick and acquires glands
· Intestine shortens.
· Pronephric kidney changes into the mesonephric kidney.

H. Physiological changes
· Liver participates in carbohydrate metabolism
· Pancreas takes up the endocrine function
· Excretion changes from Ammonotelic to Ureotelic.
· Porphyropsin changes to rhodopsin.
· During metamorphosis, the disappearance of larval organs takes place by histolysis and formation of adult organs by histogenesis.

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