Question

Which one of the following parts serves the function of primary organizer in frog?
A. Dorsal lip of gastrula
B. Grey crescent area of frog
C. Chords-mesoderm of gastrula
D. All of the above

Answer 1

Primary organizer in frog is the part of an embryo which forms pressure on adjacent parts. The frog has been kept under the category of amphibians that means it can live on both land and water, hence it becomes important as well as easy to study its life cycle as well as development from an egg to tadpole stage and then to an adult.

Complete answer:
The egg of a frog is about 2mm in diameter at the moment of ovulation surrounded by two accessory membranes along with plasma membrane. The fertilization occurs externally that means outside the body, it is mainly monospermy that is only one egg fuses with the egg at a time. The egg pronucleus and sperm pronuclei fuse to form the zygote; this process is called ‘Amphimixis’. On one side a crescent like structure appears which is in grey colour this area is called grey crescent. It has appeared on the opposite side of sperm entry. Grey crescent region in future will become the posterior side and the other one will be the anterior side of the embryo and its main function is that of an organizer. First cleavage bisects it (grey crescent) into two equal halves which further represents the medium plane of the embryo in future. Now when we particularly talk about the ‘gastrulation’ rather than discussing about the complete development of an embryo it is the process of formation of hollow gastrula from blastula, few blastomeres that are found near grey crescent are pushed inwards to form a slit, opening of this slit is called blastopore and the dorsal region of blastopore is called the dorsal lip and the lower edge is called ventral lip. Converged cells in the blastopore involute slowly and then diverge towards the roof of the archenteron, this process is called divergence. Thus an involuted cell further develops into chorda mesoderm.

Hence we can say that the correct option is (D) All of the above.

Note: During the gastrulation phase the very first cells which form the dorsal lip are endodermal cells, these are the only cells which later become pharyngeal cells. The cells that cover the dorsal lip are called mesoderm cells. Pronucleus is the nuclei of the sperm or an egg which are involved in the process of fertilization.