DEVELOPMENT OF ELASMOBRANCH FISHES. 99 



At a stage slightly younger than K an important change 

 takes place in the epiblast. 



From being composed of a single layer of cells it becomes 

 two cells deep. The two layers appear first of all anteriorly, 

 and subsequently in the remaining parts of the body. At first, 

 both layers are formed of flattened cells (PL x. fig. 8, and xi. fig, 

 9) ; but at a stage slightly subsequent to that dealt with in the 

 present chapter, the cells of the inner of the two layers become 

 columnar, and thus are established the two strata always 

 present in the epidei*mis of adult vertebrates, viz. an outer layer 

 of flattened cells and an inner one of columnar cells\ 



The history of the epiblast in Elasmobranchs is interesting, 

 from the light which it throws upon the meaning of the nervous 

 and epidermic layers into which the epiblast of Amphibians 

 and some other Vertebrates is divided. The Amphibians and 

 Elasmobranchs present the strongest contrast in the develop- 

 ment of their epiblast, and it is worth while shortly to review 

 and compare the history of the layer in the two groups. 



In Amphibians the epiblast is from the first divided into an 

 outer stratum formed of a single row of flattened cells, and an 

 inner stratum composed of several rows of more rounded cells. 

 These two strata were called by Strieker the nervous and 

 epidermic layers, and these names have been very generally 

 adopted. 



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

 though eventually they partially fuse together, there can be 

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

 where such can be distinguished''^, is derived from the epidermic 

 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 there 

 also at first fuse together^, though the epidermic layer probably 

 separates itself again, as the central epithelium of the spinal 

 canal. The lens and auditory sac are derived exclusively from 



^ The layers are known as epidermic (liorny) and mncous layers by English 

 writers, and as Hornschicht and Schleimschicht by the Germans. For their 

 existence in all Vertebrates, vide Leydig Ueber allgemrhie Bedpchungen der 

 Ampldhien, p. 20, Bonn, 1876. 



■^ Vide Leydig loc. cit. 



3 Vide Gotte Entwickhivgagpschichte der Hike. 



