EARLY DEVELOPMENT OF THE PIGEON'S EGG. 249 



blastoderm advances, the cells individualize and separate from 

 each other. The whole thickened margin is a syncytium. It is 

 an embryonic region whose depth is measured from the vitelline 

 membrane to the lowest limit of the periblast. There is not an 

 ectoderm extending over this region to the extreme margin of 

 the blastoderm. The cells next to the vitelline membrane, are, 

 of course, flattened against it, but their lower border does not 

 give the character of an epithelial layer. Only on the side of 

 this syncytium toward the center of the blastoderm do the cells 

 next the vitelline membrane form a true epithelium. From this 

 thick, syncytial border region cells individualize centrally, and 

 form two layers — (i) an upper layer, the ectoderm, and (2) a 

 lower layer of loosely arranged cells. 



It is not difficult to explain the presence of yolk-granules 

 in the cells of the upper layer because these cells, like all others 

 in this region are derived from the periblast in which the yolk- 

 granules are abundant. 



The section whose margin is shown in Fig. 16 presents two 

 layers of cells in the central part, which roof over a large cavity 

 filled with fluid. The marginal part of the cavity is shown in 

 the figure. Periblast nuclei are everywhere under the blastodisc 

 except in the nucleus of Pander. From them are derived the 

 nuclei which are, in later stages of incubation, in the germ wall 

 of the area opaca (His) and in the Randwulst. I shall not at 

 present attempt to discuss these later stages, but in a more com- 

 plete paper, I shall present further evidence in support of this 

 conception of the germ wall and shall describe it in other than 

 transverse sections, and shall also describe the formation of the 

 vascular layer. 



Summary. 



1. The supernumerary sperm nuclei migrate into the potential 

 periblast and disappear between ten and twelve hours after fertili- 

 zation. 



2. The position of the cleavage cavity is indicated by the first 

 horizontal cleavage. 



3. After the disappearance of the sperm nuclei, the marginal 

 cells open peripherally and the periblast becomes organized with 

 nuclei derived from the cleavage nucleus. 



