EARLY DEVELOPMENT OF THE PIGEON'S EGG. 247 



represented in any of the figures by His (5). Moreover, none of 

 his figures through the germ wall of the hen's egg show the ex- 

 treme margin of the blastoderm. The nuclei in the unsegmented 

 part in Fig. 16 are periblast nuclei, and their history can be 

 traced back through each preceding stage of development to a 

 period about eleven or twelve hours after fertilization when nuclei 

 from the marginal cells pass into the periblast. Indeed, such a 

 history of the nuclei may be retraced through the figures of this 

 paper — Figs. 16, 15, 14, 13, 11, 10, 9, 6. 



As development proceeds from such a stage as is represented 

 in Fig. 9, the zone of junction established by the marginal cells 

 between the blastodisc and the periblast travels outward and the 

 blastodisc increases in diameter as cells are added to its margin 



Fig. 15. Margin of a transverse section of pigeon's egg about 32 hours after fer- 

 tilization. The periblast nuclei were all in this section. One nuclear nest is shown. 

 Notice additions from the periblast to the segmented part. 



from the periblast. Cells are organized around the superficial 

 periblast nuclei and sisters to these nuclei are left deeper in the 

 unsegmented periblast. Thus the extreme margin of the blasto- 

 disc is thin, Figs. 13, 14 and 15. But later, the deeper sister- 

 nuclei are enclosed in cells and so the blastodisc thickens up 

 under that part which had been only one layer of cells in depth. 

 But, meantime, the thin margin has advanced over the yolk, by 

 addition of cells from the periblast. However, there comes a 

 time a few hours before laying (Fig. 16) when the margin thickens 

 up. This, I think, is the condition described by His (5), "Am 

 umbebruteten Keim sind die Zellen der unteren Keimschicht von 

 denen der oberen nicht allzusehr verschieden. In dem Randtheil 

 eines unbebriiteten Huhnerkeimes gehen obere und untere Keim- 

 schicht in einander iiber und sie sind nahezu gleich dick. Die 

 untere, lockerer gefiigt als die obere, ist eher etwas schwacher. 

 . . . Dotterkorner finden sich auch in Zellen der obern Schicht, 

 obwohl nicht sehr reichlich." 



In other literature it is said that the lower germ layer forms a 



