The Development of the AlHgator 239 



In Figure 3^ there is no sign of the medullary 

 groove, though the ectoderm (ec) is still much thick- 

 ened in the middle line. The section passes, pos- 

 terior to the notochord, through the anterior edge 

 of the ventral opening of the blastopore (blp). 

 The mesoderm (mes) is here again continuous with 

 the entoderm, around the edge of the blastopore, 

 but is distinct from the ectoderm. 



Figure 3/ represents the third section posterior 

 to the preceding. The blastopore, which passes 

 upward and backward through the blastoderm, is 

 seen as an enclosed slit (blp) . It is surrounded by a 

 distinct layer of compactly arranged cells contin- 

 uous with the thickened ectoderm (ec) above, with 

 the thin entoderm (en) below, and laterally with 

 the gradually thinning and scattering mesoderm 

 (mes). 



Figure 3^7 is the next section posterior to the one 

 just described. It passes through the dorsal 

 opening of the blastopore (blp) , which appears as a 

 deep, narrow cleft with thick ectodermal borders. 

 The three germ layers are still continuous with 

 each other, though the connection of the entoderm 

 with the other two is slight. The sections poste- 

 rior to this one will be described in the next stage, 

 where they have essentially the same structure 

 and are better preserved. 



Figures 371 and 30 are sagittal sections of an 

 embryo of about the stage under discussion. In 

 both figures the head-fold is seen as a deep loop 



