DEVELOPMENT. 



663 



of the pellucid area; they are not joined with the hypoblast. The inter- 

 mediate 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 primi- 

 tive 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 of flattened cells 



FIG. 447. 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). 



which forms the hypoblast proper, and an upper of several layers of 

 stellate cells, the mesoblast. 



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

 Balfours description has been chiefly followed. It differs somewhat 

 from that which has been given in previous editions of this book. The 

 mesoblast was described as arising from the hypoblast, together with 

 some of the large formative cells, which migrate by amoaboid movement 

 round the edge of the hypoblast (Fig. 488. M), and no difference was 

 made in the formation of the mesoblast in the primitive streak and else- 

 where. 



Now appears in the middle line extending forwards from the primitive 



