AVES. 129 



the sides. The part of the mesoblast, which I believe to be derived 



Fig. 97. Transverse section through the front end of the primitive 

 streak of a blastoderm of the same age as fig. 96. 

 pv. primitive groove; ni. mesoblast; ep. epiblast; hy. hypoblast; yh. yolk of 

 germinal wall. 



from the primitive hypoblast, can generally be distinctly traced. In 

 many cases, especially at the front end of the' primitive streak, it 

 forms, as in fig. 97, a distinct layer of stellate cells, quite unlike the 

 rounded cells of the mesoblastic involution of the primitive streak. 



In the region in front of the primitive streak, where the first trace of 

 the embryo will shortly appear, the layers at first undergo no important 

 changes, except that the hypoblast becomes somewhat thicker. Soon, 

 however, as shewn in longitudinal section in fig. 98. the hypoblast 

 along the axial line becomes continuous behind with the front end of 

 the primitive streak. Thus at this point, which is the future hind 



7^rs 



sailPiipigfi!»fflwp3S' 



'1 



\ 



>ty 





Fig. 98. Longitudinal section through the axial line of the primitive 



STREAK, and THE PART OF THE BLASTODERM IN FRONT OF IT, OF AN EMBRYO CHICK 



somewhat younger than fig. 99. 



pr..s. primitive streak; ep. epiblast; hy. hypoblast of region in front of primitive 

 streak; n. nuclei; yk. yolk of germinal wall. 



B. E. II. ^ 



