GASTRULATION IN THE PIGEON S EGG. 



267 



of the dorsal lip representing the " homotypical " halves of the 

 future embryo. 1 



A reconstruction of the blastoderm represented in Figs. 1 1— 1 3 

 is shown in Fig. 14. The over-growth (6>) and both zones of the 

 germ-wall (Kand Z) completely encircle the blastoderm. Just 

 anterior to the inner edge of the posterior germ-wall is the mass 

 of cells (R) which is thick in the center, but gradually thins out 



Fig. 14. A diagrammatic reconstruction of the blastoderm represented in Figs. 

 11-13. See Fig. 6 for the significance of the lettering. R, mass of cells, or the 

 deeper cells of fused halves of the dorsal lip. X 27.2. 



towards the sides. 2 Anteriorly it is continuous with the ento- 

 derm, into which it is differentiating as the latter grows forward 

 through the subgerminal cavity. At this stage the entoderm 

 [E) still ends with a free edge anteriorly, but its lateral margins 

 are united with the yolk-sac entoderm at S. The entoderm does 



L This view does not differ essentially from that advanced for the chick by Rauber 

 ('76), Whitman ('78 and '83) and others — a view hitherto not supported by experi- 

 mental workers, mainly because they experimented upon stages which were too far 

 advanced. 



2 This mass probably corresponds to what Roller ('Si) called a " Sichel." 



