672 PHYLUM CHORDATA : CLASS AMPHIBIA 



of the egg to form the ventral Up of the blastopore. The 

 bhistopore now becomes reduced, by the ingrowing of its 

 margins, to a small circular area which appears white, the 

 colour being due to a plug of yolk-cells which almost 

 obliterates its opening. The whole egg now rotates back- 

 wards through a little more than a right angle, so that the 

 blastopore is carried up into the position previously 

 occupied by the first trace of its dorsal lip. The blastopore 

 now marks the posterior end of the embryo. The archen- 

 teron has by this time greatly enlarged, and has pushed 

 the segmentation cavity almost out of existence. The 



^ N EC 



Fig. 386. — Longitudinal vertical section of frog embryo, shortly 

 before closure of blastopore. — After Ziegler's model and 

 Marshall. 



F.B., Fore-brain ; EC, ectoderm ; N., notochord ; S.C., canal of spinal 

 cord ; NE., neurenteric ranal ; B., blastopore ; M., mesoderm cells ; 

 Y., yolk-laden cells ; jViV., mesenteron ; P., beginning of pituitary 

 invagination. 



embryo elongates slightly, but the mass of yolk-laden cells 

 which lie on the floor of the gut prevents the body acquiring 

 at once the fish-like shape. 



Along the mid-dorsal line the usual neural plate forms the 

 medullary canal. At the posterior end this communicates 

 with the archenteron for a time by the neurenteric canal. 

 Internally, a diff^erentiation of endoderm forms the noto- 

 chord along the mid-dorsal line of the archenteron. At 

 each side of this lie masses of mesoderm which have been 

 split off from the endoderm. Each of these divides into 

 the primitive segments (protovertebrae) above, and the un- 

 segmented lateral plates below. The lateral plates split 

 into two layers, the splanchnic or inner investing the gut, 



