X.] THE PRIMITIVE STREAK. 323 



(Fig. 104). The axial band of hypoblast eventually 

 gives rise to the notochord. 



ep. epiblast ; me. mesoblast ; hy. hypoblast ; mg. medullary 

 groove. 



The mesoblastic elements from these two sources, 

 though at first characterised by the difference in the 

 appearance of their cells (Fig. 102, B), those of the 

 primitive streak mesoblast being more rounded, soon 

 become blended and indistinguishable from one another; 

 so that it is difficult to say to what pai-ts of the fully 

 formed mesoblast they severally contribute. 



In tracing the changes which take place in the rela- 

 tions of the layers, while passing from the region of the 

 embryo to that of the primitive streak, it will be con- 

 venient to follow the account given by Schafer for the 

 guinea-pig, which on this point is far fuller and more 

 satisfactory than that of other observers. In doing so 

 we shall leave out of consideration the fact that the 

 layers ia the guinea-pig are inverted. Fig. 105 repre- 

 sents a series of sections through this part in the guinea- 

 pig. The anterior section (D) passes through the medul- 

 lary groove near its hinder end. The commencement of 

 the primitive streak is marked by a slight prominence on 

 the floor of the medullary groove between the two diverg- 



21—2 



