DEVELOPMENT OF TELE OS T 2 II 



from their intimate relations with the yolk, are supposed 

 to subserve some function in its assimilation. 



Aside from the question of periblast, the growth of 

 the blastoderm appears not unlike that of the sturgeon. 

 From the blastula stage of Fig. 273 to that of the early 

 gastrula (Fig. 275), the changes have been but slight ; the 

 blastoderm has greatly flattened out as its margins grow 

 downward, leaving the segmentation cavity apparent at 

 SC. The rim of the blastoderm has become thickened, 

 as the 'germ ring;' and immediately in front of BP, the 

 dorsal lip of the blastopore, its thickening, as in Fig. 255, 

 marks the appearance of the embryo. In Fig. 276 the 

 germ ring, GR, continues to grow downward, and shows 

 more prominently the outline of the embryo ; this now 

 terminates at HP, the head region ; while on either side 

 of this point spreads out tail-ward on either side the indefi- 

 nite layer of outgrowing mesoderm, MES. In the stage 

 of Fig. 277 the closure of the blastopore, BP, is rapidly 

 becoming completed ; in front of it stretches the widened 

 and elongated form of the embryo. A sagittal section 

 through a late stage of the blastopore appears in Fig. 278 ; 

 with it may be compared the corresponding region of the 

 sturgeon of Fig. 256; the yolk plug, YP, of the latter is 

 now replaced by periblast, P, the dorsal lip at BP, by 

 TM, the tail mass, or more accurately the dorsal section 

 of the germ rim ; the coelenteron under the dorsal lip 

 has here disappeared, on account of the close approxima- 

 tion of the embryo to the periblast ; its last remnant, 

 the Kupffer's vesicle, KV, is shortly to disappear. At 

 TM, the germ layers become confluent as at BP in Fig. 

 256, but, unlike the sturgeon, the flattening of the dorsal 

 germ ring, TM, does not permit the formation of a neu- 

 renteric canal. 



