DEVELOPMENT OF THE OVUM. 



89 



which spreads on the inner aspect of Eauber's layer, and absorbs 

 or mixes with it, to form one layer of more or less columnar 

 cells, the epiblast or ectoderm (Fig. 67); an inner layer also spreads 

 out from the germinal group, and forms a second lining to the 

 blastodermic vesicle, the hypoblast or endoderm (Fig. 67). 



The Primitive Streak and Groove. — In the diagrammatic 

 section of the bilaminar blastoderm, given in figure 67, the hypo- 

 blast and epiblast are seen to be fused at one point, and at the 

 point of fusion the epiblast is thickened and somewhat depressed. 

 When the blastodermic vesicle is viewed from the surface, the line 

 along which the fusion of the two layers and thickening of the 

 epiblast take place is marked by the primitive streak and groove 

 (Fig. 68). At the anterior end of the groove is situated the 

 blastopore or neurenteric canal, an opening into the cavity which 

 the hypoblast encloses — the archenteron (see Figs. 67 and 75). 

 Eound the blastopore and along the primitive streak the epiblast 

 is continuous with the hypoblast. 



The Mesoblast. — The mesoblast or mesoderm, the third of the 

 primitive blastodermic layers, is produced from the margins of 

 the primitive streak (Fig. 67). At the line of junction of the 



Fig. 



edullary fold 

 medullary groove 



blastopore 



primitive groove 

 \& streak 



3. — Diagram of the Embryogenie area of a Bilaminar Blastoderm viewed from 

 above. 



epiblast and hypoblast there is a free proliferation of cells which 

 spreads out between the two primitive layers, and gives rise to a 

 third or intermediate layer — the mesoblast (Fig. 69). In lower 

 vertebrates the mesoblast is entirely produced from the hypoblast. 

 Neural Canal. — It is at this early stage, while the mesoblastic 

 cells swarm in between the epiblast and hypoblast, and while the 



