ONTOGENY 



249 



form the rod-shaped notochord. The cells covering the outside 

 of the gastrula may now be called ectoderm. The rudiment of 

 the notochord thus extends from the blastopore region forward 

 under the ectoderm and above the roof of the endodermal 



;>gc 



Fig. 153. — Schematized diagrams of gastrulation in the frog. A. optical 

 section of blastula from left side showing the location of the gray crescent, gc, 

 and the edge of the germ ring, gr, encroaching upon the yolk. B. beginning 

 of gastrulation showing the hypothetical composition of the gray crescent sub- 

 stance, according to various interpretations. C. showing downward move- 

 ment of the dorsal lip of the blastopore, dl, and the appearance of the ventral 

 lip, vl, on the opposite side of the gastrula. D. formation of gastrocoel, separa- 

 tion of notochord from endoderm. b, blastocoel; dl, dorsal lip; ec, ectoderm; en, 

 endoderm; g, gastrocoel (archenteron) ; gc, gray crescent; gr, lower edge of germ 

 ring; m, mesoderm of ventral lip; no, notochord; np, neural plate; vl, ventral lip; 

 yc, yolk cells; yp, yolk plug. Curved arrows outside figures; clockwise show 

 movement of dorsal lip; counter-clockwise show rotation of embryo. Other 

 arrows indicate direction of cell movements. 



cavity. The third germ layer, the mesoderm, originates from 

 cells lying between the ectoderm and endoderm, just within the 

 lip of the early blastopore; and as the blastopore closes, meso- 

 derm is formed throughout the extent of the blastopore. From 

 this region mesoderm grows forward on either side of the noto- 



