738 
DR. E. B. WILSON ON THE DEVELOPMENT OP RENILLA. 
relative positions, and are doubtless identical: the nucleus of No. 3 corresponds with 
one of No. 2 ; and one of No, 6 with that of No, 7. I find, counting each amphi- 
aster as two nuclei, the egg contains fifteen nuclei, and, counting the elongating 
nucleus of No. 5 as two, we have a total of sixteen nuclei, corresponding with the 
number of spheres formed at the first cleavage. 
The segmentation nuclei bear no resemblance to the germinal vesicle or egg-nucleus. 
When in the quiescent state they appear as intensely stained finely granular areas, 
shading off, insensibly, into the surrounding mass of the vitellus, and without 
enclosing membrane or nucleoli. In later stages, when the vitellus has undergone 
division, they sometimes appear as small vesicles, containing a clear substance, and a 
very deeply stained nucleolus. The yolk-granules are almost always disposed in 
radiating lines about the nucleus, but this appearance varies greatly, and is some¬ 
times scarcely discernible. I have no new observations to offer on the phenomena 
attending their multiplication, since the abundance of deutoplasm obscures the 
structure of the nuclei and amphiasters. Nuclei were, however, observed in every 
stage of division, and I will briefly describe their transformation: The nucleus 
becomes slightly elongated, then decidedly so, and the radiate arrangement of the 
surrounding granules is very marked (fig, 88). In a slightly later stage, the nucleus 
has a dumb-bell shape, with the vitelline granules radiating from each extremity 
Still later, the typical amphiaster form is attained, with two deeply stained nuclear 
areas, surrounded by very marked radiating lines of granules, and connected by a 
striated spindle. The stars then move apart, the spindle becomes attenuated. In 
later stages, during the cleavage process, the body of the cell splits into two at this 
stage oi tne amphiaster, the line of division passing at right angles to the spindle, 
near its middle point. In the unsegmented egg, the two stars simply move apart, and 
the spindle entirely disappears. 
After the division of the amphiaster is completed, the two new nuclei assume the 
ordinary appearance, and the radiating arrangement of the yolk-granules becomes 
less marked. 
The nuclei are at first situated near the centre of the egg, but as the time for 
cleavage approaches they travel towards the periphery, where the first segmentation 
spheres are to make their appearance, 
b. The cleavage process. 
The first division of the vitellus (fig. 89) consists in the formation of rounded 
prominences over its surface, of which each contains one of the nuclei derived from 
the continued division of the segmentation nucleus, and is therefore the equivalent of 
a cell (i.e., a segmentation sphere). These spheres are however entirely fused 
together, and there is at first no trace of lines of division between them. The egg is 
still a polyplast or syncytium though the vitellus is being acted upon by forces which 
tend to split it up into separate portions corresponding in number with the nuclei. It 
