Gastriilatioii in the Pigeon's Egg. 87 



pore (b), which passes beneath the dorsal lip (d) to become the 

 archenteron (ac). Directly above the archenteron is the invaginated 

 entoderm (e), and just in front of the anterior limit of this is a 

 portion of the subgerminal cavity (sg), above which the blastoderm 

 is two cells thick, but anterior to which it is three thick. Owing to 

 the obliquity of the plane of section, the wrong impression is given 

 as to the condition of the blastoderm directly in front of the central 



Fig. VIII. A diagrammatic reconstruction of a blastoderm taken thirty- 

 four hours after fertilization, or seven hours before laying. Invagination 

 has just talien place and the entoderm (E), a tongue-lilie process, is start- 

 ing to grow forward through the subgerminal cavity. As indicated by the 

 numbers, the blastoderm is thinned out to one or two cells deep in the 

 central part, while around the anterior and lateral margins it varies from 

 two to four deep. The anterior inner edge of the zone of junction is dif- 

 ferentiated into a germ wall. In this, as in the other reconstructions, the 

 ectoderm is not represented. 



part of the invaginated region. If the series had been cut parallel 

 to tlio longitudinal axis of the future embryo, a median section 

 would have been diagrammatic in clearness, that is, it would show 

 what we should expect to find in case of a true involution. The con- 

 dition of the central part of the blastoderm, however, can be inferred 

 from the photograph shown in Fig. 49. On either side of the in- 

 vaginated region the post<^n'ior ends of the sections terminate with 

 thin free margins (Fig. 16), and differ from those in the invaginated 

 area, therefore, in having no cavity posteriorly. 



