80 



EMBRYOLOGY. 



means of an intermediary cell-mass still in continuity along the suture 

 produced by the concrescence. But a separation soon takes place 



mf 



Ih 



'iuk 



ik 



mj 



l# 

 B 



Iz 



n 



usk 

 ch 



Ih 



ml? 



ik 



Fig. 4l Cross sections through the dorsal halves of three Triton larvae. 



A, Cross section through an egg in which the medullary folds (mf) begin to appear. 



B, Cross section through an egg whose medullary furrow is nearly closed. 



C, Cross section through an egg with closed neural tube and well-developed primitive segments. 

 mf, Medullary folds ; mp, medullary plate ; n, neural tube (spinal cord) ; ch, chorda ; 



ep, epidermis, or corneal layer ; -ink, middle germ-layer ; mk\ parietal, ink", visceral sub- 

 division of the middle germ-layer ; ik, inner germ-layer ; ush, cavity of primitive segment. 



along this line, inasmuch as the intermediary band of substance 

 becomes narrower and narrower, and one part of it unites wit.li the 



