STAGES G TO K. THE EXTERNAL EPIBLAST. 317 



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 (PI. 11, figs. 8, 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 

 epidermis of adult vertebrates, viz. an outer layer of flattened 

 cells and an inner one of columnar cells 1 . 



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 but 

 little doubt that the horny layer of the adult epiblast, where such 

 can be distinguished 2 , 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 cereb'ro-spinal nervous system, and there also at first fuse 

 together 3 , 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 nervous layer 



1 The layers are known as epidermic (homy) and mucous layers by English writers, 

 and as Hornschicht and Schleimschicht by the Germans. For their existence in all 

 Vertebrates, vide Leydig Ueber allgemeinc Bedeckimgen dcr Amphibicn, p. 20. Bonn, 

 1876. 



2 Vide Leydig, loc. cit. 



:! Vide Gotte, Entivicklnngsgeschichte der Unkc, 



