2020-06-12
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The second phase of gastrulation in birds is immigration (Fig.18). It consists of complicated movements of cells of the epiblast. The cells start to move from the cranial (head) edge toward the caudal (tail) edge of the embryo. Two streams of moving cells come across one another at the median line on the caudal part of the embryo. Then the streams fuse, turn and move toward the cranial part along the median line of the body, forming thickened cell cord - primitive streak. The anterior broaden part of the primitive streak is termed as the Hensen' node (the primitive knot), which is effectively the equivalent of the dorsal lip of the amphibian blastopore.
A depression that forms within the streak is the primitive groove. The primitive groove is a dip through which the cells of epiblast move deep into and migrate on both sides of the primitive streak. Initially, the cells migrate anteriorly and ventrally forming the foregut endoderm, and displace the hypoblast cells. Gradually more epiblastic cells migrate on both sides of the primitive streak intervening between endoderm and ectoderm. These cells give rise to the middle germ layer - mesoderm.
B 
