GASTRULATION 



649 



a sac, and though they have arisen neither by invagination, as 



in the lancelet, nor by overgrowth (epiboly), as in the frog, but 



by division of a single layer of cells (delamination). A fourth 



method of gastrulation by the immigration 



of individual cells is found in Hydra (p. 



100), It should be noted that in the chick 



there is at no time a true blastopore, 



leading from the exterior to the enteron, 



and that the latter is formed in a manner 



quite different from the invagination by 



which it starts in the lancelet and frog. 



fiijp. tm's. 



MESODERM AND NOTOCHORD 



Fig. 506. — A diagram of 

 a section across the 

 primitive streak of a 

 chick embryo with 

 arrows showing the 

 direction of the immi- 

 gration of the cells 

 which form the meso- 

 derm. 



Lettering as in Fig. 505. 



The primitive streak is not a morpho- 

 logical entity, but a region into and 

 through which successive cells are moving. Those which will form 

 the mesoderm spread out laterally beneath the ectoderm (Fig. 505 ; 



Epidermis. 

 Neural plate . 

 Notochord 



Primitive streak. 



Mesoderm. 



Fig. 507. — Chick embryo at eighteen hours, showing the presumptive areas. The 

 arrows show the direction of movement of the cells in gastrulation. 



