DR. E. B. WILSON ON THE DEVELOPMENT OF RENTLLA. 737 
stage of the segmentation. In some preparations, however, it does not appear, and 
sometimes it is marked in one part of a section and invisible in other parts (see 
fig. 94). 
As shown in fig. 86, the germinal vesicle lies in contact with this clear peripheral 
layer, which extends inwards slightly to meet it. In the section following that which 
is figured, the peripheral layer actually bends inwards so as to form a slight funnel- 
shaped depression leading inwards towards the germinal vesicle. Possibly this may be 
due to shrinkage ; for I have never observed such a depression in fresh eggs. More 
probably it should be regarded as a kind of micropyle through which the spermatozéon 
enters the egg. The metamorphosis of the germinal vesicle, consequent upon fertilisa- 
tion, was not followed. 
As already stated, there is a considerable interval between fertilisation and the first 
visible cleavage. Sections through the egg show that, although the vitellus is 
apparently inactive, the nuclei are rapidly multiplying. The egg, which at first 
contains a single nucleus, becomes polynuclear and passes into the condition of a 
polyplast or syncytium, each nucleus corresponding to one of the future segmentation 
spheres, as shown by later sections. 
I did not succeed in following completely the progressive multiplication of the 
nuclei, and can only assert that they become more numerous up to the time of 
cleavage, when each sphere contains a single nucleus. Many sections were obtained 
containing two nuclei, several with three, and a few in which four nuclei were visible. 
By making series of consecutive sections through the ova, it is possible to determine 
approximately the number of nuclei. I have thus observed the egg with four nuclei, 
others with four amphiasters, representing the multiplication of four nuclei into eight, 
and others containing eight separate nuclei. In others the number of nuclei is still 
greater, and in one case I was able to count sixteen nuclei, as described below. The 
nuclei do not always divide simultaneously, for I have, in several cases, observed 
eggs containing ordinary nuclei, and also typical amphiasters, with their characteristic 
spindles and star-shaped heads. In some cases an amphiaster and an undivided 
nucleus appear in the same section. In all these cases the eggs were perfectly 
spherical before treatment with reagents, and showed no sign of division. 
Fig. 87 represents a section through a spherical unsegmented egg, a few minutes 
before its fellows divided into sixteen spheres. The irregularity of form is a result of 
shrinkage which, however, affects only the external form, leaving the substance of the 
vitellus uniform, and quite free from shrinkage cavities. This section is from one side 
of the egg, and contains two distinct amphiasters. Passing inwards, the second section 
contains two nuclei, and the third one. Four nuclei appear in the fourth, and four in 
the fifth, which is represented in fig. 88. Three of the latter are simple, while the 
fourth is elongated, and apparently about to become an amphiaster. Two nuclei 
appear in the sixth section, one in the seventh, one in the eighth, and a single 
amphiaster in the ninth and last. The four nuclei of sections 4 and 5 have the same 
