Gastrulation in the Pigeon's Egg. 103 



there arc many degenerating nuclei found in the yolk lying beneath 

 the floor of the arehenteron, but in such the nuclei are in the very 

 last stages of disintegration (Fig. XIII). The absence of yolk 

 nuclei beneath the archenteron is not characteristic of the birds 

 alone, for in some of the Selachians also the same condition is found 

 (e. g., in Torpedo and Squalus). 



In the interruption of the posterior zone of junction we have 

 another line of comparison with the teleostean development ; for this 

 process is but the separation of the blastoderm from the underlying 

 periblast. The comparison will become all the more obvious when 

 we shall have shown experimentally that approximately that por- 

 tion of the margin of the avian blastoderm, beneath which the zone 

 of junction has disappeared, enters into the formation of the embryo. 

 In other words, in the teleost the entire margin of the blastoderm 

 separates from the periblast, and this whole margin (germ-ring) 

 concresces to form the embryo; whereas, in the case of the bird, 

 only about seventy to eighty degrees of the margin of the blastoderm 

 parts company with the periblast, and just about this portion of the 

 posterior edge is concerned in the process of concrescence. 



Since abnormal nuclei are found as early as fifteen hours after 

 fertilization (Harper, '04), there would seem to be some doubt 

 regarding the possibility of such nuclei being instrumental in bring- 

 ing al)Out the interruption of the posterior zone of junction. Further- 

 more, I have found degenerating yolk nuclei in eggs taken several 

 hours after gastrulation. JSTeverthelcss, there is certainly no period, 

 aside from that of gastrulation, in which they are in such abundance ; 

 and in addition to this, they are present mainly where the interrup- 

 tion takes place. The fact that such nuclei later are found gradually 

 extending anteriorly around the margin, would only indicate that 

 there was a tendency for the entire margin of the blastoderm to 

 separate from the periblast. 



c. Postgastrular Stages. 

 In eggs taken slightly later than the preceding, the entoderm is 

 found not only to have grown farther forward, but also to have spread 

 to the sides, so that its lateral margins have become united with the 



