DEVELOPMENT. 



773 



intermediate layer of cells in this position producing the primitive 

 streak is a portion of the intermediate layer or mesoblast. It is 

 formed chiefly from the epiblast, but laterally, especially in the front 

 part of the primitive streak, it appears to be derived at any rate in part 

 from the cells of the primitive lower layer. At the most anterior part 

 of the primitive streak, at the point which corresponds to the future 

 posterior end of the embryo, the three layers are all joined together. 

 The next important change which occurs is found in the hypoblast in 

 front of the primitive streak. The irregular layer of primitive cells of 

 which it is composed, split into two layers, the lower consisting of flat- 



Fig. 465. Portion of the germinal membrane, with rudiments of the embryo; from the ovum 

 of a bitch. The primitive groove, A, is not yet closed, and at its upper or cephalic end presents 

 three dilatations, B, which correspond to the three divisions or vesicles of the brain. At its 

 lower extremity the groove presents a lancet-shaped dilatation (sinus rhomboidalis) c. The 

 margins of the groove consist of clear pellucid nerve-substance. Along the bottom of the groove 

 is observed a faint streak, which is probably the chorda dorsalis. D. Vertebral plates. 

 (Bischoff.) 



tened cells which forms the hypoblast proper and an upper consisting of 

 several layers of stellate cells, the mesoblast. 



In the preceding account of the formation of the blastodermic layers, Bal- 

 four's description has been chiefly followed. It differs somewhat from that 

 which was formerly given. The mesoblast was described as arising from the 

 hypoblast, together with some of the large formative cells, which migrate by 

 amoeboid movement round the edge of the hypoblast (fig. 466, M), and no differ- 

 ence was made in the formation of the mesoblast in the primitive streak and 

 elsewhere. 



There now appears in the middle line extending forward from the 

 primitive streak an opaque line, which proceeds almost to the anterior 



