FATE OF BLASTOPORE, ETC., IX CHELOXIA. 



85 



hnve called the yolk-pliig is, at this stage, homologous y\'\th the 

 Yolk-mnss peeping through the yolk-blastopore of the Elasmobranch 

 (Woodcut XIII, r/). 



Again, if we compare Series XIII and XIV, representing the 

 sections through the tnil of somewhat more advanced chelonian 

 embryos, with Schwarz'.s Figs. 15-22 (^89) or with Ziegler's 

 Fig.s. 24, I-V, (^92) the similarity is very striking : some of the 

 sections of tlie Elasjnobrancli will pass for those of Cheloiiia. Also, 

 consider how and irliere the anus arises in Chelonia^ (Series XjII and 

 XIV) and compare it with the same event in tlie Elasmobranch* 

 (ZiECxLER Fig. da, loc. cit and ]1\lfour '78) : the two are exactly alike. 

 In botli, the proctodeum arises in a part of the primitive streak placed 

 in the ventral median line of the tail (Woodcut XIII, A). 



If a comparison is made of the stages before reaching that of 

 Figs. N and 8«, we sliall also find many things to strengthen the 

 similarity. In this sense, the change of shape in the chelonian 

 blastopore (Woodcut Y ., j). 14) aapures a new and deep significance. 

 If we examine how the blastoderm comes to surround the volk in 

 Elasmobi-anchs, and notice specially its posterior edge, we sliall find that 

 at first tlie ])osterior edge has a crescentic outline with the concavity 

 turned antcriorhj. As the Ijlnstoderm gradually spreads over the yolk 

 the concavity of the crescent is gradually reduced until at onetime, 

 the posterior edge must be straight (with the median notch at the em- 

 bryonic rim), and with still further progress, it ac(pu"res a crescent- 

 shape with the concavity turned po.s/^'/w///. (Ijalfour. Comp. Em- 

 bi-yol. \o\. ii. Fig. 30). After a certain time, that stretch of the 

 posterior eA^e nearest the embryo on each side comes to meet the 

 corres[>oiiding sti-eteh of the other side in the median line, |)roducino- 



* See the point A iu Woodcut XIII. 



