346 W. iby. R. WELDON. 
tomeres into two. In this way an equatorial band of eight 
cells is produced, with a cap of four cells at each pole. An 
ege at this stage, seen from one pole, is shown in fig. 3. The 
central yolk-mass is more evident in section than before; and 
there seems in some cases to be a well-marked difference 
between the character of the unsegmented mass and that of 
the yolk contained in the blastomeres themselves. Anextreme 
case of such difference is shown in fig. 11, where the central 
mass consists of small, highly-refracting spherules, packed 
closely together, and contrasting strongly with the larger, less 
refringent masses of the peripheral yolk. This contrast 
between the two kinds of yolk varies greatly in extent, and 
may, perhaps, be due to variations in the action of reagents. 
In the later stages it entirely disappears. As will be noticed 
in fig. 11, the development of the appearances referred to is 
frequently accompanied by a very distinct separation between 
the central yolk-mass and the surrounding blastomeres. 
In spite of Kingsley’s definite statement to the contrary, I 
feel sure that the central mass consists, both now and during 
the whole period of segmentation, of yolk containing only a 
minimal quantity of protoplasmic reticulum, and entirely 
devoid of nuclei. Kingsley’s statements, which will be dis- 
cussed below, led me to pay especial attention to the constitu- 
tion of the central mass; and I have absolutely failed to 
demonstrate the presence of central nuclei, or of central 
masses of protoplasm, at any period of segmentation. In 
sections stained with picrocarmine the contrast is so striking 
between the brilliant pink of the nuclei, the rosy colour of the 
protoplasm, and the bright yellow of the yolk, that it would 
be difficult to overlook a nucleus in the most cursory 
examination of a series of sections. 
The nuclei, as will be seen from fig. 3, do not appear on the 
surface during the stage with sixteen cells. The stage with 
thirty-two cells I have not seen; but at sixty-four cells the 
nuclei are distinctly visible on the surface of the blastomeres 
(fig. 4). 
The regular character of the segmentation is preserved until 
