440 
DE. W. H. EANSOM ON THE OVUM OE OSSEOUS EISHES. 
mention here, incidentally, that young fry, taken from the streams in November, mea- 
sure about 1" to 1-j", and their ova, which belong to Groups 2 and 3, reach about 
Young fry in June of in length, contain ova not much exceeding but some fry 
are met with in this month about 1" in length, and in all respects as far advanced as 
are some of the later hatches met with in early spring, and which do not seem to have 
grown at all during the winter. In adult females in November, the largest eggs are 
about 
3rd Group. — The smallest certainly recognizable ova measured -g^" (excluding now 
a single observation in which I met with what appeared to be still earlier ova without 
distinguishable yelk of any kind around the germinal vesicles), Plate XY. fig. 12. They 
are spherical, have a distinct ovisac lined with cells, a central, comparatively large 
germinal vesicle, a yelk of one kind only, which is solid, yellowish, refractive, homoge- 
neous, semitransparent, and is not covered by a distinctly separable yelk-sac, but has a 
smooth defined border, probably indicating its first trace (Plate XV. fig. 13). As the 
eggs grow larger, the first change noted is a faintly granular aspect of the yelk, and, 
with certain methods of examination, an appearance as if a clear substance occupied the 
centre around the germinal vesicle. 
The yelk-sac is separable in eggs measuring about and may be seen in the fluids 
on the slide as a homogeneous-looking, collapsed sac. Eggs a little larger are less 
translucent, the yelk is more granular, the free yelk-sac is seen to be furnished 
with buttons, and has the dotted structure ; these eggs measured y^g", the germinal 
vesicle measuring - 322 " • Later on, the cortical layer is seen to have the yellow droplets. 
2nd Group. — The oil begins to appear in eggs about -yg-" in diameter, at first as scat- 
tered small granules, and the whole egg is then more opaque, not only from the presence 
of the oil, but partly from its larger size, partly from the more markedly granular struc- 
ture of the cortical layer (Plate XY. fig. 14). These eggs pass into the above described 
first group, with gradual increase of the oil and grouping of the large drops. 
In both of these groups the germinal vesicle is central and globular ; nor could I suc- 
ceed at any time in making out how it became transferred to its excentric position, and 
received its lenticular form. I saw also no discus proligerus, although in all the eggs 
of the second group a food-yelk exists and escapes on rupture, apart from the formative 
yelk or cortex, and probably also in some of the later stages of the third group. 
To examine the relations of the parts of the eggs in both these groups, water is not a 
good medium, as it changes them too rapidly by imbibition ; but its action on the ovisac 
is noteworthy. It distends the contained cells rapidly, and passes through the yelk-sac 
so as to act on the yelk without forming a breathing-chamber. A solution of chloride 
of sodium, 1 per cent., is a very good medium to be used, — it exhibits well the cells lining 
the ovisac, and causes very little distension ; but a 1 per cent, solution of glycerine is 
perhaps better for taking measurements in, as it neither distends nor shrivels for some 
time, while it leaves the whole field very clear. A solution of the acetate of potash of 
the same strength is also a good medium. All these, after some minutes, permit one 
