122 



EMBRYOLOGY. 



important process. The increase which the germ-disc has undergone 

 during successive stages is indicated by dotted lines. The margin of 

 the fold, where the upper germ-layer passes over into the lower 

 layer, or the anterior lip of the blastopore, is denoted by a h; avy 

 black line. In the figures A, B, C, one 

 observes how, with the inert using extent of 

 the germ-disc, the right and left halves of 

 the blastoporic lip come together in the 



median plane 

 g . J 



| in ever-increas- 



2 ing extent, and 



| form the primi- 



3 tive groove. 



I In figs. 83 



| and 84 are pre- 



"o sented instruc- 



"= tive cross sec- 



J > tions through 



I & the primitive 



* - groove in the 



* -| first stages of its 

 . development. 

 ft < - The first shows 

 J o JS us the two lips 

 g | | of the blasto- 

 |g-g pore (fig. 83 ul), 

 % .5 separated by a 

 o g -^ small space, 

 2g~: into which 

 ^ ** ^ there projects 



from below a 



small elevation (dp) of yolk-substance, 

 containing a number of nuclei (merocytes), 

 comparable with the RUSCONIAN yolk-plug 

 in the Amphibian larva (fig. 78 dp). At 

 the lips, the upper germ-layer, a single cell thick, bends around into 

 the lower germ-layer, composed of loosely associated cells. The 

 blastopore leads into the ccelenteron, which lies between yolk and 

 germ-disc. In fig. 84 the margins of the two folds have come into 

 close contact, and have fused to form the anterior part of the primi- 

 tive streak, above which the primitive groove is still to be found. 



