76 A. RICHARDS. 



posure is without some effect, but whatever changes there may be 

 induced arc very slight if the exposure is during a resting stage. 

 Only a slight stimulation of rather uncertain nature can be 

 produced. It does not manifest itself by changing appreciably 

 the rate of division so far as hastening is concerned ; the evidence, 

 however, would indicate that later on the phase of depression 

 (see below) follows such an exposure although it may be in less 

 degree. Cytological examination shows only slight effects of 

 exposure during the resting stage on the structure of the proto- 

 plasm. 



It is probably not misleading to say that during the resting 

 stage the egg is in a state approaching equilibrium in which 

 activation is with difficulty produced. 



Recent experiments on developing eggs have in general given 

 results similar to those just outlined. Conklin ('13) found 

 abundant proof in his Crepidula experiments of the principle 

 enunciated some years ago that dividing nuclei are more easily 

 disturbed by environmental change than nuclei at rest. Koer- 

 nicke ('05) noted that after an exposure of the roots of Vicia Faba 

 and of Pisum sativnm for two days to radium the resting nuclei 

 appeared unaffected. This general result has been obtained so 

 widely that it seems unnecessary to cite further proof for a 

 position against which there is no contradictory evidence. 



Following the general rule that resting nuclei are only with 

 difficulty stimulated one would expect little result from stimu- 

 lating eggs in the germinal vesicle stage. As far as effect on the 

 rate of cell division is concerned exactly that result was obtained. 1 

 Fertilization had of course already occurred, for the sperm enters 

 Planorbis eggs at the time of laying. The egg, therefore, at the 

 time of exposure was beginning a new cycle of development, 

 caused by the entrance of the sperm, but its nucleus had not yet 

 started upon its cycle and so was not disturbed by the stimulation. 



1 (5) The eggs in this experiment were in the germinal vesicle stage at the time 

 of the exposure, which lasted 20 seconds. At the end of the exposure no change 

 could be noted. One hour and thirty minutes later the maturation divisions had 

 passed and the first cleavage furrow was making its appearance. One hour later 

 the second cleavage was nearing completion. Four and one half hours later the 

 first quartette had divided. 



(6) Eggs in germinal vesicle stage were exposed three minutes with no visible 

 effect. Four and one half hours later they were killed and fixed in the four cell 

 stage. 



