SEGMENTATION. 237 



now so small, as to be barely visible from the surface with a 

 simple lens. A section of an embryo of this stage is repre- 

 sented in PL 6, fig. 8. The section, which is drawn on the 

 same 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 T i^ 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 -^^ 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 //). 



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 seg- 

 mentation, the cells of the whole blastoderm have become 

 smaller (PL 6, fig. 9). 



The epiblast (_ep} 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. 



