122 



EMBRYOLOGY. 



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

 Z-ng successive stages is indicated by dotted lines. The margin o 

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

 layer, or the anterior lip of the blastopore, is denoted by a heavy 

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

 observes how, with the increasing extent of 

 the germ-disc, the right and left halves of 

 the blastoporic lip come together in the 

 median plane 

 in ever-increas- 

 ing extent, and 

 form the primi- 

 tive groove. 



In figs. 83 

 and 84 are pre- 

 sented instruc- 

 tive cross sec- 

 tions through 

 the primitive 

 groove in the 

 first stages of its 

 development. 

 The first shows 

 us the two lips 

 of the blasto- 

 pore (fig. 83 wfy 

 separated by a 

 email space, 

 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. 



