On the Effect of Centrifugal Force on the Frog’s Egg. 391 
vesicles surrounded by numerous astral radiations in an egg centrifuged 
one and a half hours. Fig. 9, from a region of the same egg further re- 
moved from the animal pole, represents numerous asters apparently in 
the act of division and not connected with chromosomes or nuclei. Dü- 
ring the action of the centrifuge the nuclei remain very near the animal 
pole, though mitotic figures and supernumerary asters may be found a 
little further down in the egg. If the centrifugal force is sufficiently 
strong, division of the nuclei without division of the cytoplasm occurs, 
which may or may not be foUowed by division of the cytoplasm after 
the eggs are removed from the centrifuge. Just what effect these ab- 
normalities in the process of mitosis have on the development of the em- 
bryo, I cannot determine, but it is probable that they are partly respon- 
sible for the abnormalities in development. 
In conclusion to this section, it seems probable that the abnormal 
development of centrifuged frog’s eggs may be due to one of a number of 
causes or to a combination of causes. Currents in the egg may be partly 
responsible for its abnormal development, which possibUity is rendered 
more probable by the fact that the egg of Arlacia, in which centrifugal 
force does not produce currents, is little effected in its development 
thereby, however I have given other reasons for this difference (09, 2). 
On the Effect of Centrifugal Force on the Egg at Different Stages of 
its Development. 
It has been recorded by several observers that the degree of the 
effect of centrifugal force on the frog’s egg at different periods of its deve- 
lopment is different. Konopacka studied this subject systematically, 
and found that the ratio of abnormal to normal embryos resulting, in- 
creased markedly when the force was applied after the entrance of the 
spermatozoan, and that there was a second period of increase of this 
ratio when the force was applied just before the completion of the third 
cleavage, after which the ratio decreased again. Without knowing of 
Konopacka’s results, in the spring of 1908, I performed similar experi- 
nients, which might be of interest since the centrifugal force was greater 
and acted for a shorter time (1 minute) and the results were denoted by 
the character of the most perfect embryos. The eggs of Rana pipiens 
laid by one female were divided into lots, and at the end of each ten 
minute period one lot was centrifuged with a force equal to 2771 x gravity 
for one minute. Part of each lot was then preserved for sectioning and 
the remainder placed in a dish of tap water to await depelopment. The 
highest developmental stages reached by the eggs of the various lots 
