ANATOMY

Detailed explanation-1: -A superficial layer of a fertilised egg which, in birds, is a flat disc of cells at one pole, and in insects, an outer layer of cells surrounding the yolk mass. In eggs with a large amount of yolk, cell division (cleavage) may restricted to blastoderm (meroblastic cleavage).

Detailed explanation-2: -Technically, the blastoderm is the true egg. Shortly after fertilization, the blastoderm begins to divide into 2, 4, 8 and more cells. The first stages of embryonic development have begun and continue until the egg is laid. Development then subsides until the egg is incubated.

Detailed explanation-3: -This thickened portion of the blastoderm becomes the embryonic primordium that will develop into the embryo, while the other cells of the blastoderm are designated as the extra-embryonic ectoderm (Figure 3.15).

Detailed explanation-4: -The blastodisc, also called the germinal disc, is the embryo-forming part on the yolk of the egg of an animal that undergoes discoidal meroblastic cleavage. Discoidal cleavage occurs in those animals with a large proportion of yolk in their eggs, and include insects, fish, reptiles and birds.