458 W, B. SCOTT AND HENRY F. OSBORN. 



are numerous, elongated, wedge- or spindle-shaped ; but 

 even yet there is no indication of distinct layers. 



As in the Bird, the Mammal, and the Elasmobranch Fish, 

 the epithelium lining the spinal canal does not become dif- 

 ferentiated till a considerably later period. 



As a whole the spinal cord is now a hollow cylinder with 

 very thick walls and a very small lumen. It presents a 

 transversely oval section, and is somewhat indented on its 

 lower surface by the pressure arising from the notochord. 

 The epiblast has met and coalesced ^long the middle line 

 above the canal, though a slight groove still shows the line 

 of union. 



From this time forward the outer layer of the general 

 epiblast becomes flatter and flatter, while the inner layer 

 grows more columnar. But in those parts of the skin which 

 cover the brain both layers are composed of very much 

 flattened cells (PI. XXI, fig. 13). The inner or mucous layer, 

 when once formed, is the active layer, and from it alone such 

 structures as the lens of the eye are derived. 



The primitive condition of the epiblast in Triton is an 

 extremely interesting one, presenting in a somewhat un- 

 expected manner great diiferences from that of the Frog. As 

 is well known, in the latter animal the epiblast is double- 

 layered from an extremely early period, the roof of the seg- 

 mentation cavity being formed by tAvo layers of cells, and by 

 the time of invagination there is an outer stratum of a single 

 row of flattened cells and an inner stratum of several rows of 

 rounded cells, the epidermic and nervous layers of Strieker. 

 " Both strata have a share in forming the general epiblast, 

 and though eventually they partially fuse together, there can 

 be little doubt that the horny layer of the adult epiblast, 

 when such can be distinguished, is derived from the epi- 

 dermic layer of the embryo, and the mucous layer of the 

 epiblast from the embryonic nervous layer. Both layers of 

 the epiblast assist in the formation of the cerebro-spinal 

 nervous system, and they also at first fuse together, though 

 the epidermic layer probably separates itself again as the 

 central epithelium of the spinal canal." (Balfour, loc. 

 pit., p. 99.) 



All this is very different from what we see in Triton. At 

 first the epiblast is of one layer, and so remains for a con- 

 siderable time; the mucous layer, when formed, consists of a 

 single stratum of more or less columnar cells, and the epi- 

 thelium of the spinal cord appears for the first time at a much 

 later period. In short, the condition of the epiblast, except 

 iu the last respect, is more like that of Petromyzon than that 



