MADE AT THE PLYMOUTH LABORATORY. 261 
croup of refringent granules. It does not stain so deeply as the 
nucleoli. The younger stages of it described above as seen in 
fresh preparations are not visible in the sections, but only the later 
stages in which it is situated near the surface of the ovum. 
The vitelline nucleus persists in the ovum during the development 
of the yolk. To obtain satisfactory sections of the yolked ova, as 
a rule other reagents must be used than those which succeed best 
with the ovaries in which yolk has not begun to develop. Picro- 
sulphuric acid is one of the reagents which give good results. I 
have some sections prepared with this reagent from a large plaice 
taken from the aquarium and killed in August. In these many of 
the smaller yolkless ova are collapsed and shrunken, and the con- 
nective tissue between the ova is distended and broken, but the 
yolked ova are in many cases wonderfully perfect. The stain used 
was hematoxylin, and the yolkless ova are over-stained, but in this 
respect also the yolked ova are very satisfactory. The yolk forms 
an external layer of varying thickness according to the size of 
the ege; and between it and the germinal vesicle is a layer of 
finely reticulate protoplasm. The vitelline nucleus has a shape 
which suggests that of an octopus: towards the centre of the egg 
it is rounded and has a definite outline, although it is not separate 
from the surrounding protoplasm, but continuous with it. It hes on 
the inner boundary of the layer of yolk, which consists of small 
yolk spherules. On the outer side the vitelline nucleus gives off a 
number of diverging processes which run into the yolk layer, 
becoming continuous with the protoplasmic strands which separate 
and enclose the yolk spherules. The substance of the vitelline 
nucleus is deeply stained (in hematoxylin), and in structure is finely 
granular, not as in the younger stage composed of a small number 
of refringent granules. 
As the thickness of the yolk layer increases it at lasts passes the 
vitelline nucleus, so that the latter comes to be situated entirely 
within the yolk-containing layer of the egg, and can be seen as an 
island of granular protoplasm surrounded by the yolk spherules. 
In this condition it is not so conspicuous, and is relatively smaller. 
My preparations showing this stage are from a large plaice killed in 
August, and the largest eggs in the sections are about °28 mm. in 
diameter. In these most advanced eggs there is still a layer of 
protoplasm containing no yolk between the layer of yolk and the 
germinal vesicle. 
During the development of the yolk the germinal vesicle exhibits 
changes. The chief of these is that the nucleoli are no longer 
almost always arranged in a single row at the outside of the vesicle, 
but are seen scattered in the central regions. In many preparations 
