AMPHIBIA. 



127 



nervous tube has become separated from the external skin, the 

 two layers forming it fuse together ; but there can be but little 

 doubt that at a later period the epidermic layer separates itself 

 again as the central epithelium of the nervous system. 



Both the nervous and epidermic strata have a share in form- 

 ing the general epiblast ; and though eventually they partially 

 fuse together yet the horny 

 layer of the adult epidermis, 

 where such can be distin- 

 guished, is probably derived 

 from the epidermic layer of 

 the embryo, and the mucous 

 layer of the epidermis from 

 the embryonic nervous layer. 



In the formation of the 

 organs of sense the nervous 

 layer shews itself through- 

 out as the active layer. The 



FIG. 73. DIAGRAMMATIC LONGITUDINAL 



SECTION OF THE EMBRYO OF A FROG. (Modi- 

 fied from Gotte.) 



nc. neural canal ; x. point of junction of 

 epiblast and hypoblast at the dorsal lip of the 

 blastopore ; al. alimentary tract ; yk. yolk- 

 cells ; m. mesoblast. For the sake of sim- 

 plicity the epiblast is represented as if com- 

 posed of a single row of cells. 



lens of the eye and the audi- 

 tory sack are derived ex- 

 clusively from it, the latter 

 having no external opening. 

 The nervous layer also plays 

 the more important part in 

 the formation of the olfactory sack. 



The outer layer of epiblast-cells becomes ciliated after the 

 close of the segmentation, but the cilia gradually disappear on 

 the formation of the internal gills. The cilia cause a slow 

 rotatory movement of the embryo within the egg, and probably 

 assist in the respiration after it is hatched. They are especially 

 developed on the external gills. 



Urodela. In the Newt (Scott and Osborn, No. 114) the medullary 

 plate becomes established, while the epiblast is still formed of a single row 

 of cells ; and it is not till after the closure of the neural groove that any 

 distinction is observable between the epithelium of the central canal, and the 

 remaining cells of the cerebro-spinal cord (fig. 75). 



Before the closure of the medullary folds the lateral epiblast becomes 

 divided into the two strata present from the first in the Frog ; and in the 

 subsequent development the inner layer behaves as the active layer, precisely 

 as in the Anura. 



