176 



EMBRYOLOGICAL TYPES 



of overgrowth in the centre of the grey crescent, forming the 

 dorsal lip of the blastopore. Underneath this lip is a groove 

 formed by the cells tucking in. The lips of the blastopore 

 extend right and left from the site of its first appearance. At 

 the same time the edge of overgrowth moves down towards the 

 vegetative pole, and more and more of the lighter-coloured 

 cells become covered over by the overgrowing darker ones. 



Fig. 76. — Transverse sections through the closed blastopore of Rana (A) 

 and the primitive streak of Gallus (B). 



The groove between the fused lips of the blastopore of Rana is the rem- 

 nant of the blastopore, and corresponds to the primitive groove (ps) of Gallus. 

 ec, ectoderm ; e?i, endoderm ; m, mesoderm ; all of which are continuous 

 with one another at the rim of the blastopore or primitive streak. 



Eventually the two horns of the lip of the blastopore meet on 

 the ventral side, and the blastopore is then a closed ring, formed 

 by overgrowing dark cells, and beneath which the tucking-in 

 takes place. This tucking-in is most active on the dorsal side. 

 The groove sinks deeper and deeper into the embryo, as the 

 ingrowing cells push farther and farther fonvards beneath 

 the superficial layer. The groove represents the cavity of the 



