432 
EDWIN G. CONKLIN 
centrifuging. The first cleavage appeared 4 hrs. later, the dis- 
tribution of substances to the blastomeres being abnormal in 
many cases. On the 5th day young ciliated veligers were present 
and apparently normal. On the 7th day the entire lot accident- 
ally dried up. 
Experiment 11. Eggs of Physa in the germinal vesicle and first 
polar body stages (most of the eggs had just extruded the first 
polar body, while others were still in the germinal vesicle stage), 
were centrifuged for 5 min. The stratification into gray, clear 
and yellow substances was rather incomplete, with a general 
preponderance of yellow. Many of the eggs showed a lobe of 
gray substance, though none of them were markedly elliptical, 
and all were irregular in form. At the first cleavage the eggs 
divided into two equal blastomeres, and the distribution of sub- 
stances was generally symmetrical, though unsymmetrical in a 
few cases. On the 2d day the blastulae, on the whole, appeared 
normal ; while on the 9th day all had developed into normal snails 
and some had hatched. 
Experiment 12. Physa eggs, which were pear-shaped and were 
probably in the act of extruding the first polar body, were centri- 
fuged 3i min. The subsequent development for the first 6 days 
was apparently normal, except for the distribution of the colored 
substances. The latter stages of development were normal. 
Experiment 13. Physa eggs were centrifuged for 5 min. after 
the formation of the first (and in a few cases possibly of the sec- 
ond) polar body. When removed from the centrifuge the eggs 
were elliptical in shape and the three zones were sharply defined. 
The yellow material was much less abundant than the gray, and 
the eggs oriented rapidly, with the yellow pole down. In the 
2-cell stage the distribution of the colored substances was, in most 
cases, symmetrical. On the 8th day little snails had developed 
which seemed normal, but had not yet hatched. 
Isolations. 1. A centrifuged egg was isolated in the 2-cell stage; 
one cell was gray, the other yellow. In the 4-cell stage two of the 
cells were gray and two yellow. On the 2d day a blast al a, nearly 
normal in form, had developed, but one-half was yellow and the 
other half gray. Subsequent daily examination showed that the 
