2/4 DEVELOPMENT OF ELASMOBRANCH FISHES. 



FIG. 3. 



Diagrammatic longitudinal sections of Bombinator igneus. Reproduced with modi- 

 fications from Gotte. 



Epiblast without shading. Mesoblast black with clear outlines to the cells. Lower 

 layer cells and hypoblast with simple shading. 



ep. epiblast. /./. lower layer cells, y. smaller lower layer cells at the sides of 

 the segmentation cavity. m. mesoblast. hy. hypoblast. al. alimentary cavity. 

 sg. segmentation cavity. nc. neural cavity. ,yk. yolk-cells. 



A is the youngest stage in which the alimentary involution has not yet appeared. 

 x is the point from which the involution will start to form the dorsal wall of the 

 alimentary tract. The line on each side of the segmentation cavity, which separates 

 the smaller lower layer cells from the epiblast cells, is not present in Gotte's original 

 figure. The two shadings employed in the diagram render it necessary to have some 

 line, but at this stage it is in reality not possible to assert which cells belong to the 

 epiblast and which to the lower layer. 



B. In this stage the alimentary cavity has become formed, but the segmentation 

 cavity is not yet obliterated. 



x. point where epiblast and hypoblast become continuous. . 



C, The neural canal is already formed, and communicates posteriorly with the 

 alimentary. 



x. point where epiblast and hypoblast become continuous. 



