80 



EMBRYOLOGY. 



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

 produced by the concrescence. But a separation soon takes place 



rig. 41— Cross sections through, the dorsal halves of three Triton larvEB, 



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



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



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



\Af, Medullary folds ; mp^ medullary plate ; ii, neural tube (spinal cord) ; eft, chorda ; 

 62?, epidermis, or corneal layer ; inA:, middle germ-layer ; :}iA:\ parietal, nifc^, visqeratl sub- 

 division of the middle germ-layer ; iA-, inner germ-layer ; ■usA, 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 



