DK. W. H. EANSOM ON THE OVUM OF OSSEOUS FISHES. 
477 
The contractions and oscillations resemble those which occur in the egg of the stickle- 
back, but they are more frequently irregular ; thus the sulcus may be represented by a 
circular pit, it may be a furrow parallel to the equator in either hemisphere, and may 
travel towards either pole ; or more rarely, it is a meridional one ; frequently there are 
two or three present travelling over the surface at the same time. 
At first the yelk-ball retains any position which it may have within the yelk-sac, so 
that its polar axis may be vertical, inclined, or horizontal, and these positions modify 
the direction and extent of the oscillation. Usually, about the time of the commence- 
ment of cleavage, the oil-drops tend in part to adhere to the germinal disk, and then the 
polar axis is always vertical. In this position it is easy to see that the germinal pole in 
oscillating describes a very wide ellipse, and there is at the same time a partial rotation 
on the polar axis ; but the oscillations vary as much as the contractions. This general 
description applies equally to the impregnated and unimpregnated eggs. 
After seven hours, in the impregnated eggs the germinal disk was cleft into sixteen 
regular segments. 
At the same time the unimpregnated ones showed a remarkable and very interesting’ 
lobulation of the concentrated formative yelk, a sort of irregular asymmetrical cleavage. 
This lobulation or pseudo-cleavage continued to increase, and to become more and more 
irregular. After twenty-five hours it was noted that portions of the discus proligerus 
were pinched off and appeared either as projecting buds, or as detached masses in the 
breathing-chamber. (This tendency of the formative yelk material to pinch off portions 
of its substance, may explain the so-called “Richtungsblaschen.”) Sometimes the whole 
mass of the concentrated formative yelk is pinched off, and lies free in the breathing- 
chamber, leaving a scar at the germinal pole of the yelk-ball, indicated by the collection 
of smaller oil-granules which do not separate with it, and by the ragged and radially 
puckered edges of the torn inner sac around. It is somewhat singular that, as a rule, 
from this scar no food-yelk escapes. Some hours later nearly all the unimpregnated 
eggs exhibited the separation of the discus proligerus, but no food-yelk had escaped. 
After fifty-seven hours some of them had their inner sacs ruptured, and a part of the 
fluid food-yelk had escaped into the breathing-chamber, but the diminished yelk-ball 
went on contracting as before. After seventy-four hours, decomposition having made 
considerable progress, the contractions were visible but feeble ; after eighty-three hours, 
they required the greatest care to see them at all. In other instances I found these 
movements continue for 105 hours, provided some acid clear food-yelk still remained 
in the inner sac: 
In an unimpregnated egg twenty-five hours after it had been pressed from the parent, 
the inner sac escaped, on rupture, in a very distinct and firm state ; its surface was 
marked by fine dots, having much the appearance of impressions of the inner surface of 
the yelk-sac. I examined this surface carefully for cilia, or ciliary motion, which have 
been considered a possible cause of the oscillation. I used a power of X 250 without 
finding any. 
