III.] THE PRIMITIVE GROOVE. 57 



the bottom of which, being thinner than the sides, 

 appears more transparent when viewed with transmitted 

 light. It is known as the primitive groove. Its depth 

 and the extent of its development are subject to great 

 variations. 



During these changes in external appearance there 

 grow from the edges of the cord of cells constituting the 

 primitive streak two lateral wings of mesoblast cells, 

 whiclj gradually extend till they reach the sides of the 

 area pellucida (Fig. 17). The two wings of mesoblast 

 meet along the line of the primitive streak, where they 

 still remain attached to the epiblast. During this period 

 many sections through the primitive streak give an 

 impression of the mesoblast being involuted along the lips 

 of a groove. The hypoblast below the primitive streak 

 is always quite independent of the mesoblast above, 

 though much more closely attached to it in the median 

 line than at the sides. The part of the mesoblast, which 

 we believe to be derived from the primitive lower layer 

 cells, can generally be distinctly traced. In many cases, 

 especially at the front end of the primitive streak, it 

 forms, as in Fig. 17, 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. 18, the 

 hypoblast along the axial line becomes continuous be- 

 hind with the front end of the primitive streak. Thus 

 at this point, which is the future hind end of the 



