Gastnilation in the Pigeon's Egg. 



95 



as a crescent-shaped area extending around the anterior and lateral 

 margins, is a structure hitherto not noted. It arises, however, at an 

 earlier period than this, and consists in the outgrowth of the marginal 

 cells beyond the zone of junction. 



Besides the entoderm and the germ-wall cells (at the sides) there 

 are many large yolk masses within the subgerminal cavity, and also 

 a few of the lower segmentation cells that have sunk down from the 



Fig. XI. A diagraiumatic reconstruction of a blastoderm taken thirty- 

 six hours after fertilization, or five hours before laying. It represents the 

 ectoderm as transparent. O, region of overgrowth ; Z, zone of junction ; Y, 

 germ-wall cells beneath which the subgerminal cavity has spread ; D, dorsal 

 lip of the blastopore ; PA, outer boundary of the area pellucida ; E, region 

 covered by the invaginated or gut-eutoderm. Lines drawn through GR, KF, 

 and GH represent the planes of the sections illustrated in Figs. 35, 40, and 

 41 respectively. The anterior margin of the entoderm as here represented 

 is only the average for the different lengths of entoderm as measured in 

 the sections, from which the reconstruction was made. The arrows at 

 the posterior margin indicate the direction of movement of the halves of 

 the dorsal lip. x 27.2. 



