EARLY DEVELOPMENT OF THE PIGEON'S EGG. 24I 



(i)on Ctenolabras where the origin of the periblast was first 

 accurately described. Only the twelve marginal cells of the 

 sixteen-cell stage of the teleost egg rest upon the yolk. The 

 contact with the yolk is at the inferior outer angle of the 

 cells, and this region "may be designated as the zone of junc- 

 tion " (Agassiz and Whitman) between the blastodisc and the 



Fig. 7. Photograph of pigeon's egg II hours after fertilization, 7.10 A. M. The 

 point of the arrow indicates the anterior side. 



periblast. The marginal cells of the teleost are open peripherally. 

 Now, there is to be recognized in the bird's egg a periblast 

 exactly comparable at this stage (eleven or twelve hours after 

 fertilization) with the periblast of the fish egg. We may think of 

 a potential periblast in the iinsegmented pigeon 's egg. Into this 

 the sperm nuclei migrate. 



After these nuclei disappear the marginal cells of the blasto- 

 disc open peripherally to the periblast and are directly continuous 

 beneath with the yolk. The nuclei of the marginal cells divide, 

 and some of the daughter nuclei migrate into the unsegmented 

 region, and thus the periblast " becomes cellular," to use the ex- 



