266 EXPERIMENTS AND OBSERVATIONS 
up and disappears. Hubbard considers that there is no direct 
evidence in support of the view that the yolk nucleus is the centre 
of yolk formation; he has seen no indication that it gives rise to 
yolk. When the yolk forms it is distributed uniformly about the 
centre of the egg. Perhaps Hubbard’s most important result is 
that the yolk nucleus passes to the opposite pole of the egg to that 
occupied by the germinal vesicle, and so defines the yolk pole, or, as 
it is sometimes called, the vegetative pole, long before the germinal 
vesicle has passed to the surface of the egg from the central region. 
I have not yet examined the last stages of maturation of the eggs 
of flat fishes. I have never detected the yolk nucleus in the eggs 
of these fishes after fertilization, when, if it is to be seen at all, it 
would be found in the thin layer of protoplasm which encloses the 
continuous mass of yolk. In my sections of the ova of conger 
nearly ripe, taken from females which have died in our tanks with 
enlarged and much-developed ovaries, in which ova the yolk is fully 
developed, I have not been able to detect the yolk nucleus. 
The wrinkling of the membrane of the germinal vesicle already 
mentioned is an indication of its degeneration. In the later 
stages of maturation, as the limit of the yolk layer approaches 
the germinal vesicle, the membrane gradually disappears, and_ the 
nucleoli become scattered in the reticulum of the vesicle. At the 
same time this reticulum appears to become denser, and although it 
still remains unstained it has a more solid continuous appearance. 
In eggs nearly ripe—for instance, in preparations from a plaice 
killed in January, in which some of the eggs were becoming trans- 
parent—the whole body of the ovum is crowded with large yolk 
spheres, and the germinal vesicle forms an unstained round island in 
the midst of these, containing a number of stained nucleoli. No 
membrane, separating the nucleus from the protoplasm in which the 
yolk spheres are contained, can be seen. The later changes by 
which the nucleus of the egg, and especially its nucleoli, pass from 
a central to an external position in the ripe egg when extruded I 
have not yet studied. 
The description given shows the history of the ovum in a flat-fish 
from the time of its first origin in the germinal epithelium to the 
stage in which it is almost ready for extrusion. The history is 
probably almost exactly similar in all fishes which produce pelagic 
eggs and have an annual spawning season. As I pointed out in my 
previous paper, the great majority of the eggs pass through the 
whole of this history in the course of a year, between one spawning 
season and the next. I have made preparations from ovaries in 
which spawning had just taken place, spent ovaries, with the 
following results. 
