Embryology of frog

Development of Frog:

Frog is a carnivorous organism that lives in moist places near water resources. Sexual dimorphism is well distinct in frog. It becomes sexually mature in three years. Breeding generally takes place in rainy season where male frog produces croaking sound to attract female frog. Egg of from is mesolecithical(moderate amount of yolk)and telolecithical(yolk concentrated towards one end).   

Fertilization: 

The process of fusion of male and female gamete is known as fertilization. In frog external fertilizations takes place. No true copulation takes place in frog due to absence of copulatory organ. here is sexual embrace between male and female called amplexus. During amplexus, male frog climb on back of female frog and hold by fore limb with a developed nutlical part. Female lays egg and male frog spread sperms over it. The eggs are released from ovary and passes through oviduct. The wall of oviduct secrete albuminous coat around each egg and released through cloaca into water. Sperm penetrate egg before the albuminous coat absorb water. At the point of entry of sperm, reception cone is formed in egg. Acrosome dissolve the egg membrane and sperm head enters into egg. As sperm penetrate ,small amount of ooplasm oozes out and lies beneath plasma membrane and makes pre vitellar space. The vitelline membrane becomes thicker and is known as fertilization membrane, As the sperm moves into the cytoplasm through penetration path ,The pigment is carried along with it and makes a dense pigmented region just opposite to the entry point known as grey crescent area. After penetration of sperm, the secondary oocyte undergo maturation giving mature egg and two polar bodies. Now,fusion takes place giving diploid zygote with 13 pairs of chromosome. 

Cleavage or Segmentation of zygote: 

Rapid and repeated mitotic division of zygote up to many cell blastula stage is called cleavage. Cleavage takes place after three hours of fertilization. Cleavage is holoblastic or complete but unequal.

1st division is by vertical furrow or meridonial division which passes from animal pole to vegetal pole through central axis of body giving two equal size called blastomere. 2nd division is also by vertical furrow but right angle to first one and giving four equal sized cell blastomere. 3rd division by horizontal forrow which lie just above equatorial plane giving four pigmented equal micromere towards animal pole and four bigger macromere toward vegetal pole. 4th division is by two vertical furrow which divide egg cell embryo into 16 celled stage with 8 micromere and 8 macromere. 5th division is by two horizontal furrow which divide egg cell embryo into 32 celled stage. After 5th division, the irregular division takes place and is asynchronized but the micromeres divide more rapidly than the macromeres giving mulberry leaves like appearance. This stage of ball of cell is called morula and process is called morulation. 

Blastula stage: 

As the micromeres divide very rapidly, the thickness of the cell increases due to which spherical cell becomes flattened. As a result, a space develops between micromeres and macromeres known as blastocoel. The micromeres arrange in two layers as blastoderm towards animal pole and on the sides and below the macromeres are many celled thick. This stage of embryo is called blastula and process is called blastulation. The prospective or future shape of the cell can be shown by a fate map. The entire pigmented region of animal pole represent the ectoderm which in future give rise to epidermis and neural plate. The cells of grey crescent gives mesoderm and notochord whereas the entire cells of vegetal pole give rise to endoderm. 

Gastrula: 

Transformation of blastula into 2 germinal layered embryo which involves mass migration and arrangement of cell is called gastrula and process is called gastrulation. It involves following process: 1)Epiboly: 

The micromeres are very active and divide very fast. Thus they migrate and spread over the vegetal pole. This mass migration of micromeres is called epiboly. 

2)Emboly: 

The active micromere invaginate through the dorsal lip as it continues to increase pressure on macromeres; some of the macromeres come out of the blastopore as yolk plug. The groove gradually increases in size and is called archenteron cavity which is the future alimentary canal. As the archenteron increases, blastocoel decreases and finally disappear. The opening of the archenteron cavity is called blastopore which is future anus.    Thus, frog is deuterostomic. 

3)Involution: 

Mid dorsally, there is a prospective neural plate cell with prospective ectodermal cells on the rest of embryo. Beneath the neural plate cells have chorda cells. The mesodermal cells roll inside and occupy the position on the either side of chorda cells. The endodermal cells shift inside and lies beneath the archenteron cavity. Due to the heavy shifting of macromeres, the process is called involution. Therefore, during the gastrulation two germinal layers gastrula with outer ectoderm with neural plate and inner chorda mesoderm with endoderm is formed. 

Neurulation: 

Mid dorsally the ectodermal cell of the embryo become thicker and makes the neural plate. The neural plate sink and make neural groove. The opening of neural groove is neural canal which develops into brain and spinal cord. 

Formation of Notochord: 

Beneath the neural plate has chorda cells which elongates and become hard and stiff making central axis of body called notochord which is modified into skull and vertebral column. 

Coelom formation:

The mesodermal cells lies on either side of the chorda cells which makes the mesodermal lateral plate. The lateral plate split into outer somatic layer and inner splanchnic layer. Between them is a space called coelom which is filled with the coelomic fluid. This type of coelom is known as Schizocoel. During the development somatic layer fuse with the ectoderm and makes the body wall whereas splanchnic layer fuse with the endoderm and makes gut wall. Therefore, the coelom lies between body wall and alimentary canal. After the formation of notochord and coelom, the endodermal cells are separated and they begin to move, and lies mid dorsal to the archenteron cavity. Therefore the embryo become three germinal layer with outer ectoderm, middle mesoderm and inner endoderm.