206 EMBRYOLOGY 



Gastrulation 



Gastrulation involves (1) a migration of cells into the blastocoel to form 

 a part of the endoderm (Fig. 120) ; (2) a concentration of cells along the 

 midline of the blastoderm to form a primitive streak (Fig. 121); (3) a mi- 

 gration of cells from the sides of the primitive streak to form the mesoderm 

 of the embryo (Fig. 122) ; and (4) the formation of a head process at the 

 anterior end of the primitive streak with the subsequent differentiation of 

 the notochord from this process (Fig. 121). 



The migration of endoderm cells into the blastocoel appears to be equiva- 

 lent to the involution of some of the endoderm at the dorsal lip of the early 

 amphibian gastrula. Indeed, until about 1937, descriptions of gastrulation 

 in the chick blastoderm pictured a transitory dorsal lip. At present, however, 

 embryologists agree that an actual opening, a blastopore, is not present and 

 that the presumptive endoderm cells migrate from the surface of the blasto- 

 derm into the cavity of the blastocoel and there arrange themselves into a 

 layer of endoderm, as illustrated in Figure 120. 



The formation of the primitive streak in the midline may be compared 

 with the formation of the lateral lips of the blastopore in the amphibian 

 gastrula (see Fig. 37). Both the primitive streak and the lateral lips develop 

 as a result of migration of cells toward the midline of the gastrula. This 

 migration' is illustrated in Figure 122, which shows by means of arrows the 

 movement of cells from the surface layer into the thick primitive streak and 

 from the primitive streak out into the lateral mesoderm. There may possibly 

 also be some movement of cells from the primitive streak into the endoderm. 

 Thus the primitive streak may represent a mass of relatively undifferentiated 

 cells which are able to differentiate into mesoderm, endoderm, or notochord. 

 It is also possible, however, that the mesoderm, endoderm, and notochord cells 

 are already differentiated in the primitive streak and sort themselves out into 

 the proper layers by mutual attraction of like cells and repulsion of unlike 

 ones. Cells in general show a certain specificity with regard to their union 

 with other cells. The specificity may be in some property of the surface of 

 the cell. 



If we assume that a specificity for adhesion of like cells exists in the cells 

 of the primitive streak, it follows that the endoderm cells will unite only with 

 endoderm cells already formed and that mesoderm cells will not enter the 



