EFFECTS ON EGGS OF FRESH WATER PULMONATES 431 
speed. Either the yellow or the gray pole floated up, though more 
had the yellow pole down than up, i e., they oriented as in 
earlier experiments, but more slowly. The polar bodies were at 
the yellow pole, or the gray pole or anywhere between the two, 
i.e., the axis of stratification formed no constant angle with the 
chief axis of the egg, and consequently the eggs did not orient 
while on the centrifuge. In general the distribution of the yellow 
and gray materials to the first two blastomeres was very abnormal 
and in some cases the form of the cleavage was abnormal also. 
On the 12th day 13 snails were normal, 8 were abnormal, and 
14 were dead. 
Experiment 8. Lymnaea eggs were centrifuged 10 min. just 
before the first cleavage. The separation of substances was not 
as complete as in earlier stages, and the eggs oriented very slowly 
if at all. After 10 days many were dead, a few were unhatched 
but living. After 12 days 9 of the latter were dead, and 2 living, 
but abnormal and unhatched. 
Experiment 9. Lymnaea eggs, at the time of the first cleavage 
(some eggs divided, others undivided) were centrifuged 10 min. 
The egg substances separated into yellow and white zones the 
latter composed of the clear and gray substances. Orientation, 
with the white pole up and the yellow down, took place very 
slowly, if at all. In many cases the yellow substance was par- 
ticularly dense around the nuclei and spindles The white sub- 
stance covered about seven-eighths of the egg, and within it 
were many yolk granules; there was no finely granular gray sub- 
stance as was the case when eggs were centrifuged in the matur- 
ation stages. On the 3d day a gastrula was formed, nearly 
normal in form; on the 4th day ciliated embryos were present; 
on the 9th day many young snails were more or less abnormal ; on 
the 13th day none had hatched and many were abnormal; on 
the 16th day 4 were normal and had hatched, 40 were more or 
less abnormal and unhatched. 
Experiments 10 to 22 were made on the eggs of Physa ancil- 
laria Say. 
Experiment 10. Eggs centrifuged 12 min. in the germinal ves- 
icle stage, the latter being present in the clear substance after 
