118 J. Thos. Patterson. 



tion of concrescence, for several able papers dealing with the problem 

 have appeared from time to time since His first clearly stated the 

 theory (Semper, '7G ; AVhitman, '78 and '83 -Kanbor, '76 ; Kollmann, 

 '85; Ryder, '85; Minot, '90, and others). It is sufficient to state 

 here my conclusion, that concrescence is the method by which the 

 avian embryo takes its rise. The conclusion is supported not only 

 by experimental evidence, but also by the structure of such blasto- 

 derms as showm in Fig. N,L, as well as by that of the rare ones 

 (Whitman, '83). In fact, it matters not from what angle we ap- 

 proach the problem, the conclusion is the same, namely, that the axial 

 material of the avian embryo is derived from the fused lateral parts 

 of the blastoporic lip. 



The anterior limit to which concrescence is operative in the 

 formation of the avian embryo is another problem, but it would seem, 

 from the result of Experiment IV, that at least that portion of the 

 embryo which lies posterior to the primary fore-brain is formed by 

 concrescence. This is in accord with the experimental results of 

 Peebles ('04) and Kopsch ('02) ; especially those of the latter, who 

 maintains that all of the embryo except the pre-chordal head area 

 arises directly from the primitive streak material (that is, from 

 material that is formed by concrescence) . 



In this paper I have endeavored to establish two main points 

 with reference to avian development: (1) that the gut-entoderm is 

 formed by invagination; (2) that concrescence is the method of 

 embryo formation. If I have been successful in establishing these 

 two points, it follows that the early development of the birds can be 

 brought into complete hannony with that of other vertebrates ; 

 for although differences do exist, yet they are those for which 

 comparative embryology has an explanation. Indeed, in the avian 

 development the differences have been brought about very largely 

 as a result of the enormous accumulation of yolk within the ovum. 

 Even concrescence itself has been made necessary as a result of this 

 accumulation, and for that reason it is a process that is to be regarded 

 as coenogenctic rather than as palingenetic. If concrescence is con- 

 sidered as a secondary process, we ought not to expect to find it in 

 the embryo formation of those vertebrates that have ova practically 



