AMPHIBIA. 



lOo 



general epiblast; and though eventually they partially fuse together 

 vet the horny layer of the adult 

 epidermis, where such can be 

 distinguished, is probably de- 

 rived from the epidermic layer 

 of the embryo, and the mucous 

 layer of the epidermis from the 

 embryonic nervous layer. 



In the formation of the or- 

 gans of sense the nervous layer 

 shews itself throughout as the 

 active layer. The lens of the 

 eye and the auditory sack are 

 derived exclusively from it, the 

 latter having no external open- 

 ing. The nervous layer also 

 plays the more important, part 

 in the formation of the olfactory 



M, 



sack. 



The outer layer of epiblast- 

 cells becomes ciliated after the 

 close of the segmentation, but 



FIG. 73. DIAGRAMMATIC LONGITUDINAL 



SECTION OF THE EMBRYO OF A FROG. (Modi- 

 fied from Onitte.) 



in', neural canal; x. point of junction of 

 ppiliList and hypoblast at the dorsal lip of 

 the blastopore; a I. alimentary tract; yk. 

 yolk-cells; in. mesohlast. For the sake of 

 simplicity the epiblnst is represented as it' 

 composed of a single row of cells. 



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, pre- 

 cisely as in the Anura. 



The mesoblast and notochord: Anura. After the disappear- 

 ance of the segmentation-cavity, the mesoblast is described by most 

 observers, including Gotte, as forming a continuous sheet round the 

 ovum, underneath the epiblast. The first important differentiations 

 in it take place, as in the case of the epiblast, in the axial dorsal 

 line. Along this line a central cord of the mesoblast becomes sepa- 

 rated from the two lateral sheets to form the notochord. Calberla 

 states, however, that when the mesoblast is distinctly separated from 

 the hypoblast it does not form a continuous sheet, but two sheets one 

 on each side, between which is placed a ridge of cells continuous 

 with the hypoblastic sheet. This ridge subsequently becomes sepa- 



