258 OUTLINES OF CHORDATE DEVELOPMENT 



the yolk through the germ wall. Thus the endoderm is in- 

 voluted from only a limited extent of the margin of the blas- 

 todisc. The region where ectoderm and endoderm become con- 

 tinuous is the rim of the blastopore; therefore we may say that 



rr\ n 



m.n. 



FIG. 96. Diagrams illustrating the idea of confluence (concrescence) as 

 applied to the chick. From Lillie (Development of the Chick). The central 

 area bounded by the broken line represents the area pellucida; external to this 

 is the area opaca, showing the germ wall (G.W.), zone of junction (Z.J.), and 

 margin of overgrowth (M.O). m.n., Marginal notch. 



the rim of the blastopore here is a short crescent at the pos- 

 terior side of the blastula. The blastoporal opening is conse- 

 quently represented by a space, really only virtual, between 

 this and the contiguous periblast and yolk. Closure of this 



