DE. W. H. EANSOM ON THE OVUM OE OSSEOUS FISHES. 
439 
In still more advanced eggs, i. e. those of full size, with concentrated, grouped oil-drops, 
although still within their ovisacs, no trace of germinal vesicle could be found on repeated 
and careful examination ; so that its disappearance precedes the escape of the eggs into 
the oviduct. The most careful and prolonged examination of the substance of the discus 
proligerus, more particularly of that part of it which had so recently contained the 
germinal vesicle, failed to show any trace of the germinal spots, or any other change in the 
structure or reactions of the matter of the discus proligerus, after the germinal vesicle 
had vanished, which could be looked on as due to its disappearance. In face of this 
negative result, however, it may be mentioned that in cases where I watched the germinal 
vesicle escape through a rupture in the yelk-sac into the surrounding fluid, whether 
water was present or not, it often happened that the vesicle was ruptured and its contents 
escaped ; and when this did occur, I frequently could not see the spots among the sur- 
rounding materials if, perchance, they were lost sight of for a few minutes. This was 
doubtless due, in part, to the fact that all the objects in the field were in a state of con- 
stant change ; still, the changes which occur in the germinal spots are such as might 
distinguish them from the only visible elements of the formative yelk which can be com- 
pared with them, viz. the yellow droplets ; for these latter vacuolate, fade, and disappear 
in water, while the former vacuolate and become darker in outline and do not disappear, 
at least for a very long time, in water. Perhaps the saline or other constituents of the 
yelk-ball may have a solvent action on the germinal vesicle or spots ; but this point, 
which might be submitted to experiment, I had not an opportunity of determining. 
The ripe or nearly ripe ovarian ova have their ovisacs decidedly thinner at that part 
which covers the germinal segment. These eggs, placed in water, speedily imbibe it, and 
become faintly opalescent. Yacuolation soon appears in the matter of the cortical layer, 
beginning at the germinal segment, and in ten minutes the yellow drops disappear, and a 
slight interval appears between the yelk-sac and the outer surface of the yelk-ball — 
a true breathing-chamber. In one or two cases, where some rupture or injury had 
taken place, a partial concentration of the formative yelk also took place, but this was 
exceptional. 
c. Earlier ovarian ova . — An adult female fish, taken from the natural haunts in the 
month, of June, after she has deposited her first batch of eggs, and in which a second is 
ripening, may be used conveniently to examine the developing ovarian ova in all their 
earlier stages. Three principal groups may then be made out with the naked eye ; 
1st, large, nearly ripe, semitransparent, yellowish ova, the oil grouped more or less ; 
2nd, medium-sized creamy tinted opaque ones, with oil scattered ; 3rd, smallest, colour- 
less or whitish, semitransparent ones without oil-drops. The two latter groups alone 
remain to be described, and I shall examine them chiefly with reference to their mode 
of growth. 
There is no advantage gained in the study of the earliest distinguishable ova by taking- 
young fry ; for adults in the autumn, winter, or early spring contain the smallest visible 
ova as easily observable as in young fish of y in length, and not a month old. I may 
3 o 
MDCCCLXVII. 
