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19. Growth and development eLearn.Punjab

Gastrulation : It is characterized by the movement and rearrangement of cells in the
embryo. During gastrulation, the blastoderm splits into two layers: an upper layer of
cells called epiblast, and a lower layer of cells called hypoblast. The epiblast is mainly
presumptive ectoderm and mesoderm (Fig 19.7). The hypoblast is mainly presumptive
endoderm because hypoblast cells grow outward over the surface of the yolk, then
downward around it to form the endodermal lining of a yolk sac. At this stage, the central
cells of blastoderm can be separated from the yolk, under these central cells a pool of
fluid develops, raising them off the yolk and giving the area a translucent appearance
- the area pellucida, while the peripheral part of the blastoderm where the cells lie
unseparated from the yolk is termed as area opaca, the white area that transmits
light. The upper layer of the blastoderm consists of the presumptive mesoderm and
ectoderm.

Notochord and Mesoderm Formation

In the chick, the mesodermal cells do not invaginate as in amphibians, but migrate
medially and caudally from both sides and create a mid line thickening called primitive
streak (which grows rapidly in length as more and more presumptive mesodermal
cells continue to aggregate in the middle. All this results in the change of shape of
blastoderm, (it changes from circular to pear shaped).

Animation 19.2: Mesoderm Formation
Source & Credit: UNSW Embryology

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