606 W. J. SOLLAS. 



fifth blastomere sometimes lying in the centre of the ovum 

 surrounded by the other four, and sometimes in a zone with 

 them (fig. 6), when a small segmentation cavity is seen in 

 the centre. At a very early stage a structureless blastema, at 

 the most very finely, or scarcely at all, granular, makes its 

 appearance. At first, as in stage 4, it merely forms a thin 

 layer round the periphery of the ovum, becoming thicker at the 

 junction of the segments, and sometimes extending for a 

 variable distance between them. It then stains comparatively 

 deeply with reagents. As development proceeds it increases in 

 quantity and takes a less decided stain, and in the final stages 

 of segmentation it exists as a clear, structureless matrix, which 

 forms the outer boundary of the segmented mass, and in which 

 the segments lie completely and separately immersed. 



After the stage of five segments, seen in section, it becomes 

 increasingly difficult to determine from sections the number of 

 segments in the total ovum, and I will content myself with 

 adding on this point that after seven, eight, and nine segments 

 appear in section the number increases rapidly, till it soon 

 becomes too great to count. A segmentation cavity is the ex- 

 ception, not the rule ; the only instances I have met with are 

 shown in figs. 12 and 13, and even here the cavity is partly 

 occupied by blastomeres. In my sections a segmentation cavity 

 does not normally occur. This is the more remarkable since 

 Barrois, Schulze, and Metschnikofi" all represent a segmentation 

 cavity as of constant and normal occurrence. Barrois and 

 Schulze differ, however, in details, the former finding a 

 segmentation cavity already at stage 8, the latter not till the 

 stage of sixteen segments has been reached. 



The blastomeres are from the first remarkably granular, so 

 that for some time they have the appearance of being a col- 

 lection of yolk granules, each separately and clearly defined. 

 The granules, at first large, gradually become smaller, rounder, 

 and less apt to stain as the blastomeres multiply and diminish 

 in size, till when the latter are reduced to 0-008 mm. in diameter 

 they form the least instead of the most conspicuous part of the 

 cell contents, while the nucleus and nucleolus, till then only 



