16 J. GRAHAM KErvR. 



thickened rim, which at first I called the growing edge of 

 the epiblast. Further investigation showed, however, that 

 the chief characteristic of this rim is not its growth, which 

 is comparatively small, but the fact that it represents the 

 mass of small cells on which the roof of the segmentation 

 cavity rested at its margin. The thin two-layered epiblast, 

 in fact, from this rim for a considerable distance is nothing 

 else than the persistent roof of the segmentation cavity. 

 This is shown to be the case by the fact that within a short 

 distance of the rim one frequently finds the small blastomeres 

 beneath the epiblast retaining their rounded form with 

 chinks between, or we may even find the segmentation cavity 

 still present as a continuous slit. 



What spreading of small cells over the large yolk- cells does 

 take place is brought about by addition to the margin of 

 small cells cut off from the yolk. This is well shown by 

 sections such as that in fig. 13, where there can be no 

 question of true epibole or sliding of the small cell layer 

 over the surface of the yolk-cells.^ 



The slight extent of the movement over the yolk of the 

 small-celled margin at the point opposite the blastopore rim 

 is of importance as providing a nearly fixed point in the in- 

 terpretation of sagittal sections. The evidence of these sec- 

 tions is, on the whole, that the dorsal roof of the archenteron 

 is formed mainly by backgrowth of the dorsal lip, and as the 

 medullary plate at its first appearance is practically coin- 

 cident with the extent of the archenteron, Lepidosiren 

 is brought into line with the Selachians, where almost the 

 whole of what is commonly called the "embryo" is formed 

 from a similar backgrowth. 



The Segmentation Cavity. — I now return to the con- 

 sideration of the segmentation cavity, which was left at a 

 period when it was beginning to be encroached upon by 

 the bulging wall of the archenteron. The further oblitera- 

 tion of the segmentation cavity, although it takes place 



• By an error the word " epibole " was used in my former |)a))er at, one 

 place (p. 322) wlien delamination was actually meant. 



