418 J. CI.'AHAM KKK'l;. 



of Lcpidosiren, in 80 far as they relate to the skin and 

 certain organs associated with it. tSome strnctnres which 

 ought logically to be described now have been deliberately 

 omitted : such are the external gills, which I propose to 

 consider at the same time as the branchial clefts. I also 

 make no attempt to describe the various organs with an 

 equal degree of fulness. In regard to some, Avhicli I have 

 found specially interesting, I give a fairly detailed account ; 

 in regard to others I am content, to outline the main 

 features. 



I have endeavoured to avoid obscuring my description by 

 going into masses of minute detail, feeling that by so doing I 

 should often be trespassing in regions where individual varia- 

 tion, and the "probable error" of observatiou^ make results 

 useless if not actuullv misleading. 



The Genet^al Epidermis. 



It has already been shown that the first part of the 

 epidermis to assume a fairly definite shape is that covering 

 the ventral surface of the embryo, which is simply the 

 persistent roof of the segmentation cavity. The epidermis 

 covering the dorsal surface of the body, on the other hand, 

 develops later. It has also been shown that during gastru- 

 lation, in Lepido siren, the dorsal lip of the blastopore is 

 composed of a mass of undifferentiated cells, showing no 

 distinction into layers. In this Lepido siren differs from 

 Ceratodus, where, as Semon points our, the epiblast is 

 marked off by a distinct split right back to the lip itself. 

 Elsewhere than at the blastopore lip epiblast is formed by 

 delamination- from the large yolk-cells underneath. By 

 Stage 14, when the process of gastrulation is finished, the 

 embryo is already covered uniforndy by a definite stratum of 

 epiblast composed of two layei'S of closely apposed llattened 

 cells, exce])t just in the li]) of the blasto])ore where the 

 germinal Inyers ai'c^ still, ami will ivMunin for some time 



