THE PHYSIOLOGY OF FERTILIZATION 157 



(1913) has, however, been able to show, by means of 

 hybridization experiments, that the cortical changes in 

 the sea urchin are not dependent on penetration. He 

 found that the sperm of Asterias would cause the for- 

 mation of fertilization membranes in the eggs of Stron- 

 gylocentrotus purpuratus in hyperalkaline sea-water, but 

 only a fraction of them segmented. Cytological obser- 

 vation showed that the spermatozoon penetrated only 

 a similar fraction, presumably the ones that segmented. 

 In others there was external action of the spermatozoon 

 alone. The eggs which had not been entered by the 

 spermatozoon could be caused to develop by treatment 

 with hypertonic sea-water. It is therefore probable 

 that it is a general rule of fertilization that the sperma- 

 tozoon sets the cortical changes of the egg in operation 

 before penetration. 



Similarly the activation of the egg by means of parthe- 

 nogenetic agents has been shown in certain cases to be 

 clearly divisible into two phases. This is shown most 

 clearly by Loeb's famous method of inducing develop- 

 ment in sea urchin eggs, which consisted of a brief 

 treatment with butyric acid or some other cytolytic 

 agency, producing the cortical changes alone, which 

 required to be supplemented by after-treatment with 

 hypertonic sea-water or certain other agencies to induce 

 complete development. Loeb explained this result by 

 supposing that the cortical changes in themselves have 

 something of excess in them, leaving the egg in a sickly 

 condition, and that this effect was counteracted by the 

 second treatment. Whether this is a correct explanation 

 or not we need not now inquire (see chap, vii), but we 

 should note that the transition from the first to the 



