D) diskoblastula; e) blastocyst.

The egg cells (and hence the zygote as well as blastula) is always asymmetric, having an animal pole and a vegetal pole. The upper hemisphere is the animal pole; the lower hemisphere is the vegetal pole. The vegetal pole draws its name from its inactivity relative to the lively animal pole. In some animal groups the vegetal pole contains large yolky cells that divide very slowly.

Completeandequal cleavage of the oligolecithal egg of Amphioxus results in a simple hollow sphere of cells enclosing a cavity – blastocoele. This sphere is termed a celoblastula.

In result of complete unequal cleavage of the mesolecithal egg of frog it is formed a blastula with an eccentric blastocoele surrounded by small blastomeres at the animal pole and by larger blastomeres at the vegetative pole. This blastula is called amphiblastula.

During incomplete and unequal cleavage of the telolecithal egg of the chick, the yolk does not divide. Cytoplasmic division are limited to the animal pole, and bilaminar disk shaped mass of blastomeres, surrounding the yolk, is produced. A cleft between the laminae is homologous to the blastocoele of lower living forms’ blastula. A subgerminal cavity between blastomeres and main yolk mass is formed shortly thereafter. It is filled with nutritive liquefying yolk. This blastula is called diskoblastula.

Complete unequal cleavage of the secondary isolecithal egg of eutherian mammal results in a compact collection of cells calleda morula(20-50 cells). Morula begins to absorb uterine fluid forming a central cavity. The blastocyst, as it now known, consists of a peripheral layer of blastomeres forming the trophoblast, with a mass of cells at one aspect, the polar trophoblast, bulging into the central lumen and known as the inner cell mass, or embryoblast from which the embryo proper arises.

GASTRULATION

The next stage of embryogenesis is a process of transformation of blastula into gastrula, the embryo, consisting of 3 primary germ layers:

· ectoderm (externally)

· endoderm (internally) and

· mesoderm (between them).

Gastrulationis rearrangement of the cells by migration and division. Gastrulation is highly variable in vertebrates so the different types will be described. Four main mechanisms of gastrulation can be distinguished in animals of the phylum Chordata (Fig.17):

· invagination;

· epiboly;

· delamination;

· immigration.