Gastrulatiou in the Pigeon's Egg. 119 



wanting in jolk ; and as matter of fact a majority of the investigators 

 on the development of Amphioxns maintain that there is no con- 

 crescence, as does Conklin ('05) also for the Ascidians. Eycleshy- 

 mer ('02), as a result of his experimental studies on the Amphibian 

 egg, concludes also that concrescence is a secondary process. He 

 says, "that in those Amphibia which approach most nearly the 

 holoblastic type, as Rana, Bufo, Acris, and Chorophilus, the greater 

 portion of the embryo is formed through differentiation in situ and 

 overgrowth, concrescence being confined to a limited region at the 

 caudal end of the embryo. In those "forms like l^ccturus in which 

 there is a marked meroblastic tendency, due to the relative increase 

 in the amount of yolk, a lesser extent of the embryo is formed through 

 differentiation m situ, while there is a corresponding increase in the 

 extent of the embryo formed through concrescence, or coalescence of 

 the lateral margins of the blastopore." Again, in his concluding 

 paragraph he writes that "there is every reason for maintaining that 

 differentiation in situ is the primitive method of embryo formation, 

 concrescence being a secondaiy process which has progressed pari 

 passu with the increase of yolk material."^^ 



Owing to the close affinities existing between birds and reptiles, 

 we should expect to find many points of comparison in their modes 

 of development. Although many Avriters have pointed out the simi- 

 larities existing between the two modes, yet, judging from the results 

 obtained in the study of the pigeou, it would be of the greatest 

 interest to be able to trace the origin of the "Primitive Plate of Will" 

 to the margin of the blastoderm, and thus to establish a more exact 

 comparison between the two forms. 



Summary. 

 The main points brought out in this paper may be stated in the 

 following brief summary : 



1. Gastrulatiou in the pigeon's egg is preceded by the thinning 

 out of the thickened blastodisc. The thinning-out process begins at 

 about twenty-one hours after fertilization, and consists in the crowd- 



'^Loc. cit., p. 353. 



