EFFECTS ON EGGS OF FRESH WATER PULMONATES 425 
coming first and the latest last. It will be understood without 
further mention that in this account Physa refers to Physa ancil- 
laria Say; and Lymnaea to Lymnaea catascopium Say. 
Experiment 1. A large egg-mass of Lymnaea was taken just 
after it was laid, and centrifuged for 10 min. in the germinal 
vesicle stage. After centrifuging the eggs were found to be 
slightly elongated or elliptical (fig. 23), the narrower end being 
central, and the broader end distal (peripheral) in the tubes. The 
egg substances were separated into three very distinct zones, gray, 
transparent (clear), and yellow; the gray substance was finely 
granular in structure and occupied the narrower (central) end of 
the egg; the clear substance was almost perfectly transparent and 
constituted the middle zone ; while the yellow material contained 
yolk spherules as well as yellow pigment and occupied the broad 
end of the egg. The gray and clear materials together consti- 
tuted about one-half of the egg substance, the gray forming about 
three-eighths, and the clear one-eighth, while the yellow sub- 
stance made up the other one-half. In all cases the germinal 
vesicle lay in the clear zone. The yellow zone was so much 
heavier than the other two that it turned to the lower side 
before it could be examined under the microscope ; it was there- 
fore necessary to tilt the microscope into a horizontal position, 
with the stage and slide in a vertical position in order to see 
these three zones. Half an hour after the eggs were removed 
from the centrifuge they all became pear-shaped, or unequal 
dumb-bell shaped; the gray substance occupying the small end, 
the yellow the large end and the clear substance lying in the 
neck between the two (fig. 25). In many cases this neck be- 
came so constricted that the two ends became nearly or quite 
separated. One hour and fifty minutes after the eggs were cen- 
trifuged the first polar body appeared, usually on the clear zone 
(fig. 26), but sometimes on the edge of the yellow or gray zone ad- 
joining the clear one. At the same time the clear zone was seen 
to be less sharply separated from the gray and the yellow than was 
the case immediately after centrifuging. The eggs still oriented 
as before with the yellow pole down and the gray pole up. Five 
hours after centrifuging the eggs had passed into the 2-cell 
