436 
EDWIN G. CONKLIN 
The experiments on the eggs of Physa were less detailed and 
exact than those on the eggs of Lymnaea. Several of the Physa 
experiments were made early in my work before I had fully appre- 
ciated the importance of detailed records as to the exact stage of 
the eggs at the time of centrifuging, the relative quantities and 
weights of the different substances, the positions of the polar 
bodies with reference to the different zones, and the precise num- 
ber of normal as compared with abnormal embryos. Furthermore 
the eggs of Physa were more easily injured than those of Lymnaea, 
so that the length of time during which the eggs were centri- 
fuged was varied considerably in the hope of finding the most 
favorable period, and these facts must be taken into account in 
comparing the results of these different experiments. On the 
whole, however, a comparison of the two tables shows that the 
effects of centrifuging were essentially similar in the two genera. 
CONCLUSIONS 
The principal results of these experiments is to show that im- 
portant changes take place in the ooplasmic substances during the 
interval between the first maturation and the first cleavage. 
These changes concern the relative quantities and weights of the 
three principal substances, and the dissimilar effects of centrifug- 
ing at different periods. These experiments also throw light 
upon questions of the polarity and symmetry of the egg and the 
potency of the ooplasmic substances. 
1. The relative proix)rtions of gray, clear and yellow sub- 
stances undergo great changes during the period between the first 
maturation and the first cleavage. Before the first maturation 
the yellow substance composes at least one-half of the entire 
egg; just before the first cleavage it composes only about one- 
eighth of the egg. The clear and gray substances, which together 
constitute about one-half of the egg in the earlier period, form 
seven-eighths of the egg in the later period. In early stages the 
yellow subsiance contains the yolk spherules and, in strongly 
centrifuged eggs, consists almost entirely of these; after the dis- 
solution of the germinal vesicle much of the yolk disappears and 
