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DE. W. H. EANSOM ON THE OVUM OE OSSEOUS FISHES. 
The Trout (Salmo fario). 
In ripe ova expressed from the fish I found the micropyle, December 1854, by care- 
fully turning the egg over before water was applied ; then by drying the surface some- 
what, it was visible with the naked eye, but more easily with a lens (Plate XVI. fig. 24). 
At first it corresponds to the centre of the discus proligerus or germinal pole, but after 
the egg has been in water a few minutes, even when not fertilized, water enters, and 
the formative yelk which is at first as in Gasterosteus, a complete cortical layer, con- 
centrates, and is collected into a nipple-shaped heap at the germinal pole; and from 
having attached to it some oil, it always floats uppermost, the yelk-ball being free to 
move in the now distended yelk-sac ; so that the correspondence between the micropyle 
and the centre of the discus proligerus ceases. The terminal opening measures g- 3 y 
across, and the funnel or pit at its mouth 
The formative yelk, the discus proligerus with a deep central pit to receive the micro- 
pyle, the clear food-yelk, and the group of oil-drops, are all essentially the same as in 
Gasterosteus. 
These eggs, pressed from the parent into water, stick to the dish for a time, but if 
first left exposed to the air for a little while, do not. This was not explained. They 
formed a breathing-chamber by imbibition, but no active protoplasmic contractions 
were seen. 
The Salmon (Salmo salar). 
These ova were examined in January 1855. They have a micropyle precisely similar 
to that met with in the egg of the trout, and the general structure of the egg is the 
same. The yelk-sac is very well suited for examining the dotted structure, especially 
after prolonged immersion in water, in which it retains its structure for four months at 
least ; the details of its structure are essentially the same as in Gasterosteus. 
These eggs, like those of the trout, imbibe water when not impregnated. If uninjured, 
they remain in water without apparent change, at least forty-three days. If injured, 
the inner sac often ruptures, and then the yelk coagulates, by the action of the water. 
If kept for the same time in damp moss, they decompose, become foetid and alkaline, 
and then, if crushed in water, do not coagulate, the salt of ammonia produced keeping 
the albumen b in a state of solution. The inner sac thus seems, when intact, to resist 
the passage of osmotic currents through it. 
I tried to test this in the following manner : — 
Eight clear ova which had been kept without change in water for twenty days were 
well shaken in an empty bottle for a minute or two ; then distilled water was added, 
and all became opaque at once, showing no longer a breathing-chamber, the inner sac 
being ruptured. The water was found to be acid, and contained an organic substance, 
which, when incinerated, left an alkaline ash containing chlorides and phosphates. 
The chemical reactions of the yelk. — In the three species of Salmonidse which I have 
examined, the yelk reacts similarly, but it differs somewhat from that of most other 
osseous fishes, in having a larger proportion of a peculiar variety of albumen precipitable 
