Egg is an apparatus containing female gamete. For nourishment of developing embryo, it contains a nutritional compound called yolk. Yolk is a complex mixture of proteins, lipids (fats), carbohydrates, vitamins and minerals.

Eggs are classified according to the amount of yolk present in them. Different categories of eggs are as follows:

The egg containing moderate to large amount of yolk. These eggs are of two types:

These eggs contain moderate to large amount of yolk which generally concentrates towards the lower half or vegetal pole of the egg. For example eggs of Amphibians.

These eggs have got a large or enormous amount of yolk generally separated from egg’s white. For example eggs of birds and reptiles.

A single female frog lays hundreds of eggs in water which are held together by a jelly like substance. The cluster thus form is called Frog’s Spawn.

A single egg of frog is round or spherical measuring about 1to2 millimeters in diameter. It is externally covered with three gelatinous layers. These transparent layers act as an effective convex lens to converge maximum sunlight to produce necessary temperature beneath these layers lies the vitelline membrane along with Perivitelline Space. The outer and superior or upper half portion of egg is pigmented (containing melanim) appears black, while the lower half is whitish in colour. The black pigment serves to absorb sunlight to produce necessary heat during Embryo Genesis. The black half of egg contains more cytoplasm than the lower yolk containing half. For this reason upper half is metabolically active and called Animal Pole. The lower yolk containing half is called Vegetal Poles. The egg as a whole is a single cell.

Female frog lays egs in water over which the male frog sheds sperms. As a rule a single sperm is capable of fertilizing a single egg. When a sperm touch the egg, it causes some physiological change in the egg by which the egg does not permit any other sperm to enter. Now, the head of the sperm bores its way and enters into the egg leaving its tail behind and outside. Here the sperm head becomes rounded and now called male pronucleus after completing into second meiotic division. Similarly the nucleus of egg completes its 2^nd meiotic division and forms female pronuclei. Now the two haploid pronuclei unite to form zygote nucleus which is Diploid (2n) in nature. Thus fertilization is completed.

After fertilization the pigment of the egg moves upward on the side lying opposite to the Reception Cone i.e. the point of entry of sperm in the egg, the bare area thus formed is called The Grey Crscent extending up to half way around egg.

As cell division advances, a cavity appears inside the cellular mass nearer to animal pole called Blastocoel and the hollow sphere of cells is called Blastula. Micromeres divide more rapidly than the megameres and soon overlap part of the megameres forming a band called Germ Ring. The cells lying at Gray crescent divide more rapidly than rest of the cells.

The process of Gastrulation involves three types of Movements of cells namely:

1. Epiboli
2. Emboli
3. Involution

With the beginning of Emboli, the cells of the srufac eare displaced along the Dorsal lip to take their definite position for the formation of organs. This massive movement of different cells to their definite positions is called Involution.

The cells of Dorsal lip migrate and turn inward along the ceiling of archenteron forming Chorda Mesoderm.

Archenterone expands dorsally and laterally, the floor of it is made up of small yolk cells while the lateral walls are composed of micromeres. During the course of its expansion the blastocoel gradually diminishes. Megameres tend to lie along the ventral lip and floor archenteron.

Due to shifting and distribution of cells and expansion of Archenterone with diminution in Blastocoel, the centre of gravity of Gastrula also shifts. The animal pole which was formally located on the top now comes to lie antero-ventally shifting nearly 120° .

After completion of Gastrulation, Blastopore closes completely forming a small longitudinal groove open at the ends. The ventral opening closes and forms the anal pit while the dorsal aperture remains open and forms Nementeric Canal. At this moment the gastrula becomes Pear shaped and composed of an outer layer of micromeres called Ectoderm. The invaginating cells lining the archenterone separate out from other layers of cells to form Endoderm.

The cells between these two layers delaminate and differentiate to form Mesoderm. Along the Mid Dorsal Line the cells of Mesoderm form a slid rod like structure called Notochord. The Mesodermal cells grow on both the sides of Notochord and finally, after forming two complete sheets, meet at the Mid-Ventral Line. Thus the Embryo becomes three layered which are called Primary Germinal Layers.

It gives rise to Epidermis, cutaneous glands, nervous system, parts of sensory organs, enamel of teeth, Internal lining of Cloaca and Mouth, Nasal Mucosa.

It forms mucosa of midgut, liver, spleen, pancreas, lung, urinary bladder and thyroid glands.

It gives rise to body muscles, connective tissue, dermis, heart, kidneys, Reproductive organs, blood vascular system, Notochord and bones.

Formation of bodily organs and systems by the germinal layers is called organogenesis. Few are given below:

In ectodermal cells of Mid Dorsal Line becomes differentiated and form a thickened plate known as Neural Plate. The edges become much thicker and fold outward forming New Folds. These folds grow mid dorsally and converge to fuse, enclosing a canal inside known as Neural Canal. After fusion of folds, the canal is changed into a neural tube. Anteriorly the tube forms three hollow swellings, which form fore brain, mid brain and hind brain. Rest of the tube forms spinal cord. This Notocord lying below the neural tube between the folds of mesoderm gradually forms Vertebral Column.

In the elongated Embryo, invagination of ectoderm occurs below the anterior end forming a pit known as Stomodaeum, the future buccal cavity. Similarly invagination of ectoderm occurs at closed blastopore posteriorly to form Proctodaeum or the future Cloaca. Stomodaeum and Proctodium open interiorly into Archenterone forming a continuous tube. The middle portion of this tube i.e. Mesenterone differentiate into different parts forming Alimentary Canal. Buds grow outward from mesenterone at different levels, which give rise to lung, liver and pancreas. The anterior part of Mesenterone just near its junction with stomodaeum forms 5 pairs of Pharyngeal Pouches.

Pharyngeal Pouches are important embryonic structures. First pair of pouches forms the tympanic membrane, Estachian tube and middle ear. The remaining 4 pairs of pouches open to the exterior and forms external gills later on.

Within 48 hours after fertilization, the embryo becomes much developed that it can survive independently. During this period following changes take place at and after hatching.

• Pear-shaped body becomes elongated and differentiated into head, trunk and tail.
• Body becomes externally covered with Epidermis, which develops Dorsal and Ventral fins of Tail. Now the embryo hatches from the egg and swims in water in search of weed. It is now called Tadpole Larva.
• Anterio-ventrally, a V-shaped sucker or cement gland develops, which helps in attachment with the water weed.
• Posteriorly laying tail becomes elongated and muscular with V-shaped Myotomes. It also has got Fins for swimming.
• Pharyngeal open to the exterior through which three pairs of external gills develop for Respiration.
• Primary vesicles finally form Fore, Mid and Hind Brain.
• A primitive tubular S-shaped heart also develops.
• Stomodaeum opens to form mouth with horny Jaws.
• Rudiments of eyes and ears develop.
• A long watch- spring like alimentary canal develops with differentiated organs like Oesophagus, stomach and intestine. The tadpole starts feeding upon waterweeds.
• Proctodaeum opens to form cloaca.
• Pronephric kidneys are developed and start functioning. At this stage tadpole is a fish like actively swimming creature.
• A fold of skin from the 1^st gill clefts began to grow backward and ovon covers the external gills. This fold of skin is called Operculum which change external gills into internal gills.
• Rudiments of hind limbs appear on both sides of Cloacal apperture while forelimbs appears just behind the head below operculum.
• Head becomes Triangular and Prominent with two bulging eyes.

In metamorphosis of Frog changes taking place can be grouped into two types, which are given below:

1. Morphological Changes
2. Physiological Changes

The two main types of Morphological Changes are:

1. Regressive Changes
2. Progressive Changes