FATE OF BLASTOPORE, ETC., IX CHELONIA. 99 



considerable progress in their development. Nevertheless, few will doubt 

 that the two are homologous, when we examine their sections, or the 

 position of the neurenteric canal with respect to the streak, etc. Of the 

 two, there can not be any doubt that the "[process seen in Chelonia 

 has retained a more primitive character, and ought to serve as a key 

 in unravelling various questions in regard to the primitive streak in 

 Birds. So far as I am aware, nobody lias yet discovered a structure 

 in Aves comparable with the mass which I have called the yolk- 

 plug ; but this may be due to the fact that noljody has till now 

 looked for it. If it exists at all, it must be iu a rudimentary 

 condition, and unless hooked at in the liglit of what takes place in 

 Keptilia, is most likely to be overlooked. Ijut I believe that future 

 investigations will throw a great deal of light on tlie point. The 

 posterior end of the primitive streak in Birds is notoriously variable, 

 and nobody has made a systematic study of it up to the present 

 time. There is one figure in existence which makes one quite 

 hopeful. I refer to Fig. 6, PI. VIII of Kupffer's classical work 

 ( 82). It is the figure of a sparrow embryo with two mesoblastic 

 somites. At the posterior end of the medullary canal there is figured 

 a triangular mass which the author has marked pr. This can be 

 neither more nor less than the yolk-plug at such a stage as Fig. 2 of 

 the present article. 



There is one piece of observation made on the fowl which goes 

 flatly against my views advanced above. AYiiitmax ( 83) has described 

 an al)normal f)r]n of the chick luastoderm in which the primitive 

 streidv is pi'olonged posteriorly to the edge of the blastoderm. One 

 naturally concludes that this prolongation represents that coalesced 

 edge of the non-embr^^onic portion of the blastopore in Elasmobrancliii 

 (e /, Woodcut XIII) which connects the embryo with tlie yoUv- 

 blastopore. This presupposes tlie fact that the yolk-mass of the 



