DEVELOPMENT OF ELxVSMOERANX'H FISHES. 43 



epiblast and lower layer cells are very different. At tins part, 

 throughout the whole extent of the embryonic rim, the epiblast 

 is reflected and becomes continuous with the lower layer cells. 



The lower layer cells form, for the most part, a uniform 

 stratum in which no distinction into mesoblast and hypoblast 

 is to be seen. 



Both the lower layer cells and the epiblast cells are still 

 filled with yolk spherules. 



The structures at the embryonic rim, and the changes 

 which are there taking place, unquestionably form the chief 

 features of interest at this stage. 



The general relations of these parts are very fairly shown 

 in PL III. fig. 5, which represents a section passing through the 

 median line of the embryonic region. They are however more 

 accurately represented in PL iv. fig. 5 a, taken from the same 

 embryo, but in a lateral part of the embryonic rim ; or in 

 PL IV. fig. 6, from a slightly older embryo. In all of these 

 figures the epiblast cells are reflected at the edge of the 

 embryonic rim, and become perfectly continuous with the hypo- 

 blast cells. A few of the cells, immediately beyond the line 

 of this reflection, precisely resemble in character the typical 

 epiblast cells; but the remainder exhibit a gradual transition 

 into typical lower layer cells. Adjoining these transitional cells, 

 or partly enclosed in the corner formed between them and the 

 epiblast, are a few unaltered low^er layer cells {in), which at this 

 stage are not distinctly separated from the transitional cells. 

 The transitional cells form the commencement of the hypoblast 

 (hy); and the cells (m) between them and the epiblast form 

 the commencement of the mesoblast. The gradual conversion 

 of lower layer cells into columnar hypoblast cells, is a very clear 

 and observable phenomenon in the best specimens. Where the 

 embryonic rim projects most, a larger number of cells have 

 assumed a columnar form. Where it projects less clearly, a 

 smaller number have done so. But in all cases there may be 

 observed a series of gradations betw^een the columnar cells and 

 the typical rounded lower layer cells \ 



1 When writing my earlier paper I did not feel so confident about the mode 

 of formation of the hypoblast as I now do, and even doubted the possibihty of 

 determining it from sections. The facts now brought forward are I hope suffi- 

 cient to remove all scepticism on this point. 



