256 DEVELOPMENT OF ELASMOBRANCH FISHES. 



posterior end of the blastoderm the cells are flatter and broader ; 

 and the layer terminates at the non-embryonic end of the 

 blastoderm without exhibiting the slightest tendency to become 

 continuous with the lower layer cells. At the embryonic end of 

 the blastoderm the relations of the epiblast and lower layer 

 cells are very different. At this part, throughout the whole 

 extent of the embryonic rim, the epiblast is reflected and be- 

 comes 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 shewn 

 in PL 7, fig. 5, which represents a section passing through the 

 median line of the embryonic region. They are however more 

 accurately represented in PL 7, fig. $a, taken from the same 

 embryo, but in a lateral part of the embryonic rim ; or in PL 7, 

 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 hypoblast 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. Ad- 

 joining these transitional cells, or partly enclosed in the corner 

 formed between them and the epiblast, are a few unaltered lower 

 layer cells (;//), which at this stage are not distinctly separated 

 from the transitional cells. The transitional cells form the com- 

 mencement of the hypoblast (Jiy) ; 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 be- 



