DE. W. H. EANSOM ON THE OVUM OE OSSEOUS EISHES. 
405 
Somewhat similar movements in the eggs of the pike have been spoken of as rotations, 
but in those of Gasterostei there is no rotation on the polar axis ; as I ascertained by 
carefully watching fixed points, on each side of a spider thread, placed so as to correspond 
to the polar axis. However, it is not intended to deny the occurrence of occasional slight 
rotations on any axis of the egg, during the various and irregular contractions which 
occur. The yelk rests on the lower part of the yelk-sac, being of greater specific gravity 
than the water in the breathing-chamber, and the oscillation takes place from the point 
of rest. 
The oscillations are caused by the contractions of the yelk. Not only do they com- 
mence with the beginning contractions, and become pronounced as the latter become 
vivid, but each commencing wave is shortly followed by an oscillation. In one obser- 
vation six to and fro oscillations were counted in thirteen minutes, each corresponding 
to a travelling wave; this was during the second quarter-hour after impregnation. 
During the next half-hour, the contractions being less vivid, thirteen to and fro oscilla- 
tions, proportionally more limited in extent and rapidity, were counted, each correspond- 
ing to a wave. Gradually, as the contractions became feeble, and limited to the germinal 
surface of the food-yelk, the oscillations ceased, and the germinal pole became stationary, 
about 35° from the micropyle. In short, the contractions measure the oscillations, so 
that irregular contractions cause irregular oscillations. In one instance, where two 
waves began, on the left-hand side of the yelk-ball, so that the first had not ceased before 
the second had begun, there were two oscillations to the right, and none to the left. 
I could not always with certainty connect the direction of each oscillation with the 
position of each commencing wave, yet, as a rule, the germinal pole swung to the right 
if a sulcus appeared on the left side of the yelk-mass, and vice versa, provided the sulcus 
was in the germinal hemisphere, or near the equator ; but when the first depression of 
the surface was in the ventral hemisphere, the oscillation carried the germinal pole to 
the same side as the sulcus. The result was, however, often modified by the direction 
of the wave, as well as by the rapidity at which it travelled. 
The oscillations were influenced occasionally by oil-drops of larger size than usual, 
adhering to the germinal disk, and making it float uppermost; then, the oscillations 
were in a vertical plane, or nearly so. The yelk-ball is of nearly equal specific gravity 
throughout, as it retains any position it may be placed in, the oil-drops usually floating 
so freely in the outer portion of the food-yelk, that they move up to the top during an 
oscillation or any other movement. 
It is therefore a fair inference that the oscillations depend on the contractions, which 
by altering the form of a globular mass of nearly equal density throughout, and partly 
floated, displace its centre of gravity, and determine the movement to restore the equili- 
brium. The onward movement of the wave Avould further modify the result. 
The unimpregnated eggs, in a ripe female which had been dead forty-eight hours 
and kept moist, were a little decomposed, and in many instances had the inner sac rup- 
tured, so that the whole or a portion of the food-yelk had escaped into the cavity of the 
