MADE AT THE PLYMOUTH LABORATORY. 265 
transparent and pelagic like these I have studied. I have been 
unable to find any indication that the yolk commences first in the 
neighbourhood of the vitelline nucleus. As I have already stated, the 
yolk layer is at first wholly external to the nucleus, and there are no 
yolk-globules or granules at the side of the latter. When the forma- 
tion of granules begins it appears as a thin layer round the whole of 
the outside of the plasm of the egg, and is not thicker near the vitelline 
nucleus than elsewhere. In most ova the layer of yolk in sections is 
of uniform thickness, but occasionally it is thicker on one side than 
the other, and then the vitelline nucleus is not at the thickest part. 
In spite of the fact that the largest yolk-globules are those of the 
inner part of the yolk layer, it seems certain that the increase of 
yolk takes place by the new. formation of globules added to the 
layer on the inner side. Forming globules are seen at the inner 
edge of the layer. After the yolk layer has so increased that its 
inner border is internal to the vitelline nucleus, it is clear that the 
new formation of globules can have nothing to do with that body. 
The most recent published paper on the yolk nucleus is that by 
Jesse W. Hubbard, of Indiana University, U.S.A.* This investi- 
gator studied the eges of Cymatogaster aggregatus, a viviparous fish 
of the coast of California, and his conclusions closely agree with my 
own. Cymatogaster belongs to the family Embiotocide, which is 
allied to the wrasses. The eggs of Cymatogaster are small, *3 mm. 
in diameter, and being developed within the ovary the quantity of 
yolk in them is naturally small. The preserving reagent used by 
Hubbard was Flemming’s strong mixture, the effects of which I 
have found in my own experience to be destructive to many parts 
of the egg. Like myself, Hubbard could see no trace of the yolk 
nucleus in very young fish, in those under 4 cm. in length. It was 
present in the ovaries of specimens over 7 cm. The smallest egg in 
which the body was observed was ‘02 mm. in diameter, and it 
appeared as a cap of stained protoplasm fitting round one side of 
the nucleus. 
In a slightly larger egg the yolk nucleus is separate from the 
germinal vesicle, and it gradually moves away from the latter. 
Hubbard concludes that the yolk nucleus originates from the 
germinal vesicle not by division, but by a general extrusion 
of substance. It passes to the external region of the egg, and 
when the yolk is formed and the egg is ripe it is situated at the 
yolk pole of the egg, opposite the blastodisc. It remains visible in 
the same position in the yolk after the egg is laid, and during 
seementation until the closing of the blastopore, when it breaks 
* The Yolk Nucleus in Cymatogaster aggregatus, Proc. Amer. Philos. Soc., vol. xxxiil, 
1894. 
