80 



EMBRYOLOGY. 



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

 produced by the concrescence. But a separation soon takes place 



usk 



Ch 



mi 1 

 Ik 



Fif . 41 Grots lections through the dorsal halves of three Triton larvae. 



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



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



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

 tnf, Medullary folds ; nip, medullary plate ; 71, neural tube (spinal cord) ; ch, chorda ; 



rp, epidermis, or corncal layer ; rule, middle germ-layer ; mk\ parietal, mi j , visceral sub- 

 division of the middle germ-layer ; H; inner germ-layer ; uth, 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- 



