Gastrulatioii in the Pigeon's Effg;. 117 



bt>' 



accord with general usages; as, e. g., in the case of the lower verte- 

 brates, Amphioxiis and the fishes, where the gut-entoderm, mesoderm, 

 and chorda are all said to be involuted at the same time in the form 

 of the primary entoderm. 



This objection, however, completely disappears if the primitive 

 streak formation is regarded as a part of the gastrular phenomenon. 

 That such an explanation for the primitive streak formation is 

 fully justified is obvious when considered in the light of comparative 

 embryolog}\ Thus in Amphioxus all of the chorda and mesoderm 

 are derived from the primary invaginated layer. In Amphibians 

 the posterior part of the mesoblast is formed about the lips of the 

 blastopore, and is often spoken of as the ''peristomal mesoblast," in 

 contrast to the more anterior portion, or "gastral" mesoblast. In this 

 case, no one hesitates to consider the whole process in Amphibians 

 as that of gastrulation, because the "•gastral" and "peristomal" meso- 

 ])last are directly continuous with one another. In the case of 

 l)irds, all of the mesoblast is derived from the primitive streak, that 

 is, it is all peristomal mesoblast. In the bird, therefore, the tran- 

 sition from a gastral to a peristomal mesoblastic formation has gone a 

 step farther than in the case of the Amphibians. We have shown 

 experimentally that as the gut-entoderm is being involuted, the con- 

 crescence of the halves of the dorsal lip is also taking place. Further- 

 more, there arises later from this fused region the primitive streak, 

 or mesoblast. It is evident, therefore, that the invagination of the 

 gut-entoderm and the primitive streak formation are but different 

 parts of the same process, namely, that of gastrulation. The occur- 

 rence of a short period (from shortly after the closing of the blasto- 

 pore to the appearance of the primitive streak), during which one 

 cannot distinguish, either by sections or by surface views, the primi- 

 tive region, in no wise invalidates the above comparison. 



Hertwig ('03) has divided the process of gastrulation in the 

 amniota into two parts or phases. He, however, had little else 

 to offer for his first phnse in the bird other than Duval's work — a 

 work over which he himself casts a shadow of doubt as to correctness, 

 as is evident from the citation given in the first part of this paper. 



It is not my purpose to enter into a discussion of the whole ques- 



