474 
DE. W. H. EANSOM ON THE OVUM OF OSSEOUS FISHES. 
irregular form, while the food-yelk underwent no change of form (Plate XVIII. fig. 72). 
None of these eggs passed on to cleavage, and all ultimately had ruptured inner sacs, 
shrunken into a dark granular mass. 
Experiment 6, — Forty minutes after impregnation the last experiment was repeated, 
with a stronger but otherwise similar carbonic-acid water. The eggs were examined 
directly, and the yelk, which had been actively contracting, suddenly ceased to move ; 
the sulci were effaced without travelling on, and under observation the yelk-balls be- 
came globular, in ten minutes the germinal disk being level with the surface. About 
fifteen minutes later the germinal mass again projected above the surface of the yelk- 
ball, and the further fate of these eggs was the same as in those of experiment 5. 
Experiment 7 was the same as the last, using eggs 3 h 35 m after impregnation, at 
which time the first cleft was fully formed. They were not examined until four minutes 
after the action of the carbonic acid ; the germinal disk was then withdrawn into the 
yelk-ball in a singular manner (Plate XVIII. fig. 73). In ten minutes more it was pro- 
jecting again, but irregular in form; afterwards, by still slower steps, it was again flat- 
tened, but not drawn into the yelk-ball. While this went on the cleavage masses un- 
derwent a gradual fusion, which commenced as early as thirty minutes after closing the 
cell ; and at length, 5 h 30 m after impregnation, the yelk-ball was globular and at rest 
(Plate XVIII. fig. 74). By degrees the diffused germinal mass became darker and more 
granular, and eight hours after impregnation many of the eggs had ruptured inner sacs, 
and the contents were changed to a darkly granular mass, which consisted chiefly of the 
decomposed formative yelk-substance, with some oil-drops and granules (Plate XVIII. 
fig. 75). All the changes of figure observed after the yelk has first effaced its sulcus, 
are imperceptible in progress. 
Experiment 8 was the same as the last, using eggs 3 h 45 m after impregnation, the 
germinal disk being then cleft into four masses. The eggs were examined at once, in 
order to trace the steps of the first retraction of the germinal disk into the yelk, this 
took place under observation until the yelk-ball exhibited no prominence on its surface. 
(See Plate XVIII. fig. 79, which shows the complete retraction, and figures 77, 78, which 
show the intermediate stages : fig. 76 is given as a normal standard.) I could not ascer- 
tain that any changes of form took place in the mass of the food-yelk, and the explana- 
tion of the withdrawal of the germinal mass into the food-yelk is at present not made 
out. During the next ten minutes, by invisible movements, the germinal disk became 
again prominent, the cleavage masses being irregular in form and wanting in symmetry. 
Much more slowly after this, sinking of the germinal disk began, and gradual fusion of 
the cleavage masses ; so that five hours after impregnation, or l h 15 m after the action of 
the carbonic acid, some of the eggs had still prominent and irregular germinal disks, but 
all showed more or less fusion of the cleavage masses, and by 6^- hours after, the fusion 
was complete. It was not, however, till eight hours after, that the germinal mass was 
quite reduced to the state shown in Plate XVIII. fig. 74. Later still, in many, but not 
in all the eggs, rupture of the inner sac and dark granular precipitation occurred. 
