DEVELOPMENT OF ELASMOBRANCH FISHES. 25 



sarae scale as the section belonging to the last stage, serves 

 to shew the relative size of the segments in the two cases. 



The epiblast is now more distinct than it was. The seg- 

 ments composing it are markedly smaller than the remainder 

 of the cells of the germinal disc, but possess nuclei of an abso- 

 lutely larger size than do the other cells. They are irregular 

 in shape, with a slight tendency to be columnar. An average 

 segment of this layer measures about y^^ inch. 



The cells of the lower layer are more polygonal than 

 those of the epiblast, and are decidedly larger. An average 

 specimen of the larger cells of the lower layer measures about 

 ■^ij^ in. in diameter, and is therefore considerably smaller than 

 one of the smallest cells of the last stage. The formation of 

 fresh segments from the yolk still continues with fair rapidity, 

 but nearly comes to an end shortly after this. 



Of the nuclei of the lower layer cells, there is not much 

 to add to what has already been said. Not infrequently two 

 nuclei may be observed in a single cell. 



The nuclei in the yolk which surrounds the germinal disc 

 are more numerous than in the earlier periods, and are now 

 to be met with in fair numbers in every section (Fig. 8 ?i'). 



These are the main features which characterise the present 

 stage, they are in all essential points similar to those of the 

 last stage, and the two germinal discs hardly differ except in 

 the size of the segments of which they are composed. 



In the last stage which I consider as belonging to the 

 segmentation, the cells of the whole blastoderm have become 

 smaller (PL ir. %. 9). 



The epiblast (ej)) now consists of a very marked layer of 

 columnar cells. It is, as far as I have been able to observe, 

 never more than one cell deep. The cells of the lower layer 

 are of an approximately uniform size, though a few of those at 

 the circumference of the blastoderm considerably exceed the 

 remainder in the bulk. 



There are two fresh features of importance in germinal 

 discs of this age. 



Instead of being but indistinctly separated from the sur- 

 rounding yolk, the blastoderm has now very clearly defined 

 limits. 



