80 



EMBRYOLOGY. 



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

 produced by the concrescence. But a separation soon takes place 



mp 



ep 



Fig. 41 Cross sections through the dorsal halves of three Triton larvae. 



A, Cross section through an egg in which the medullary folda (m/) 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; 



cp, epidermis, or corneal layer; ml-, middle germ-layer; mk\ parietal, nifc 2 , visceral sub- 

 division of the middle germ-layer ; U; 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 with the 



