DR. W. H. RANSOM ON THE OVUM OP OSSEOUS FISHES. 
479 
these sulci, which passed nearly halfway across the field of the microscope, equal in this 
case to about was effaced in forty seconds. Three hours later these contractions were 
still going on, but nine hours after rupture of the egg they had ceased. During their 
continuance there was a gradual emptying of the fluid food-yelk and shrinking of the 
inner sac. 
So long as in impregnated ova any portion of the food-yelk remains uncovered by the 
gradually advancing germinal mass, its surface is constantly moving, and the waves of 
contraction are seen to pass beyond the margin of the germ, and under it, to the contained 
food-yelk. Seventy-four hours after fecundation there remains but a small area at the 
ventral pole uncovered by the advancing germ. But long after the whole of the yelk 
is covered by the germ, at least as late as the ninth day after impregnation, slow 
rhythmic contractions of the contained food-yelk are visible, producing alternate depres- 
sions of the lateral poles of the abdominal region, and consequent oscillations of the 
embryo, which have been described by Reichert*. 
On the tenth day some of these eggs were hatched. I could not see any contractions 
in the food-yelk of free embryos, but the search was not very carefully made. 
In the pike, as in the stickleback, eggs which have been allowed to remain in the 
dead parent for a certain time cannot be fertilized, even when they have undergone no 
discoverable change of structure. In the pike, after seven hours, the capacity of being 
impregnated was lost; but then a physical change had occurred, for water no longer 
passed through the yelk-sac to form a breathing-chamber. 
On the whole, I think it may be said that the inner sac is essentially connected with 
the exercise of this contractile property. It is difficult to ascribe contractility, at least 
of this rhythmic kind, to a substance so fluid as the food-yelk, in which minute monads 
can swim about freely, and in which, when escaped or escaping, I never saw the slightest 
evidence of contractility. On the other hand, except the single observation above 
related, of a retained shred of inner sac in a crushed egg, I have no satisfactory evidence 
that the inner sac alone is capable of contracting ; and that instance may possibly have 
been fallacious, for the closely connected pouch of the inner sac filled with food-yelk, 
might at each of its contractions have pulled at and moved the shred. The persistence 
of the movements of the food-yelk contained in the embryo, when one bears in mind 
that the inner sac is folded in during cleavage, and might therefore fairly be expected 
to be used up in the gradual extension of the germ over the yelk, would seem to give 
support to the notion held by Reichert f , that the substance of the food-yelk is the con- 
tractile matter. But it may be replied that the inner sac may possibly be retained on 
the surface of the contained food-yelk in the abdomen of the embryo, and to this view I 
incline. But while it appears probable that the contractile property resides in the inner 
sac, I am disposed to think that the presence on its inner surface of some of its acid yelk 
is an essential condition of its action. 
* “ Der Nahrungsdotter des Hechteies, — cine kontraktiie Substanz,” Muller's Areluv, 1857, p. 48, 
t Muller’s Archiv, loc. cit. 
3 T 
MDCCCLXVII. 
