DEVELOPMENT OF ELASMOBRANCH FISHES. 



Gl 



Diagrammatic longitudinal sections of Bombinator igneiis. 

 modifications from Gotte, 



Reproduced with 



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

 Lower layer cells and hypoblast with single shading. 



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

 of the segmentation cavity. 711. 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 ap- 

 peared. X is the point from which the involution will start to form the dors'al wall 

 of the ahmentary 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 reahty 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 segmen- 

 tation cavity is not yet obliterated. 



X. point where epiblast and hypoblast become continuous. 



C. The nem-al canal is already formed, and communicates posteriorly 

 with the alimentary. 



X. point where epiblast and hypoblast become continuous. 



