426 
EDWIN G. CONKLIN 
stage. The gray and yellow poles could still be distinguished, 
but the clear zone between them had almost disappeared, and the 
eggs no longer oriented rapidly, if at all. The yellow and gray 
poles of the egg occupied any conceivable relation to the first 
cleavage plane, with the result that one of the first two cells might 
be gray, the other yellow (fig. 35), or the gray and yellow sub- 
stances might be distributed equally to the two cells (fig. 39), or 
there might be many intermediate conditions between these two 
extremes (fig. 43). If the distribution of these substances was un- 
symmetrical in the first two blastomeres, it remained unsymmetri- 
cal throughout the whole course of development. The cleavage 
forms were usually regular and normal except for the distribution 
of the yellow and gray materials, and a blastula, gastrula and larva 
of normal form developed from every egg, though the yellow and 
gray substances might occupy any position in the embryo or 
larva. Finally after fifteen days all of the young except one, some 
40 in all, had hatched, and all were perfectly normal little snails. 
Three eggs of this lot were isolated in the 2-cell stage, being 
selected because they showed very unsymmetrical distribution 
of the yellow and gray substance. They continued to develop for 
from 3 to 15 days but all of them died before hatching. In 
view of the fact that all the other eggs in this experiment devel- 
oped into normal snails it seems probable that these isolated 
eggs failed to develop because of some injury due to isolating 
them, or to some injurious substance in the dishes in which they 
were placed. 
Experiment 2, A Lymnaea egg-mass was centrifuged 10 min. 
about 20 min. after the eggs were laid, and while all were still 
in the germinal vesicle stage. The separation of the egg sub- 
stances into gray, clear and yellow zones occurred as in the pre- 
ceding experiment. The first polar body was extruded 1 hr. and 
30 min. after the centrifuging, and in every case (37 eggs) from 
the clear zone. The eggs oriented as in the preceding experiment, 
and in order to see the three zones under the microscope it was 
necessary to tilt the stage into a vertical position. The second 
-polar body was extruded 1 hr. and 15 min. after the first, usually 
from the clear zone, though the latter had mingled considerably 
