470 
D E. W. H. E AN SOM ON THE OVUM OE OSSEOUS EISTIES. 
appeared, which were due to the contractions of the food-yelk with its covering. 
Electrolytic changes, however, appeared very readily in the substance of the cleavage 
masses, which became regularly crenate at the margin, as if composed of small glo- 
bular cells (Plate XVIII. fig. 70). The current used in this observation decomposed 
water. 
Ova nine minutes after impregnation, before visibly moving contractile waves had 
commenced, contracted to the zero-currents. 
The cortical layer of the food-yelk being carefully examined at 40 ni and at 2 h 30 m 
after impregnation, with a power of Xl95, it was found that a few scattered granules 
and yellow droplets always remained, which had not been collected into the germinal 
disk ; and these, on rupturing the inner sac by a strong current, were observed to retreat 
with it from the broken part, and to undergo vacuolation at the same time. In this 
way, better than by any other, the intimate connexion which exists between the inner 
sac and the remnants of the formative yelk where it is spread over the food-yelk, is 
shown. 
c. Heat . — Some observations were made to ascertain how far abstraction of heat 
diminished the susceptibility of the contractile material of the yelk to galvanic stimulus. 
Control experiment. — Ova were fertilized and kept, some in cells, others in capsules, 
at the temperature of the room, 58° F. After thirty-five minutes they were actively 
contracting and rotating in the usual way ; they completed the first cleft after 2\ hours, 
and the second after five hours. The germinal mass was in the fine mulberry stage of 
cleavage after twenty-four hours, and then no contractions of the yelk were seen, but 
zero galvanic currents excited them distinctly in the yelk, without causing any move- 
ments in the germinal mass. 
Experiment a . — Some of the same ova fifteen minutes after impregnation were placed 
in a chamber cooled to 45° or 48° F. : forty-five minutes after, they were contracting 
normally but languidly, and responded to the galvanic current apparently as well as did 
the ova in the control experiment. 
Experiment a '. — Some of the same ova, two hours after impregnation, were put into 
a chamber cooled to 40° F. The first cleft was not completed until 3 h 45 m after impreg- 
nation ; so that cold, even when it does not act until late in the stages which precede 
cleavage, retards its progress. 
Experiment b . — Some of the ova which had been cooled to 45°-48°F. were, one hour 
after impregnation, further cooled, so that the thermometer on the cell stood at 32° F., 
the water not being frozen. The yelk-ball became round and still, but zero-currents of 
galvanism somewhat slowly excited very distinct contractions. In this experiment, 
however, the cell was rapidly receiving heat from the stage of the microscope, as I had 
then no means of maintaining it at a constant temperature. 
Experiment c . — Some of the stock of control ova were, one hour after impregnation, 
cooled so that the water was nearly all frozen, as well as some of the eggs, which were 
then allowed to thaw. Those which had been frozen were decidedly opake, and various 
