FERTILIZATION AFTER INITIATION OF DEVELOPMENT. 26 1 



spermatozoon or by an artificial agent and, unless we assume 

 such a relatively restricted set of reactions, we have as yet no 

 explanation to offer for parthenogenesis. The principal issue 

 at hand is, can an egg of Arbacia that has been once activated 

 by hypertonic sea-water be fertilized or reactivated by a sperma- 

 tozoon of the same species? 



The results obtained from this study show conclusively that a 

 treatment of the sea-urchin egg with hypertonic sea-water for 

 an optimum length of time e. g., that in which one obtains the 

 highest percentage of development, both in regard to cleavage 

 and to larval production leads to changes in the physiological 

 state of the egg that result in the loss of its capacity for fer- 

 tilization. Spermatozoa readily enter the eggs but they are 

 unable to cause a reactivation of the egg; they appear only as 

 inert bodies within the cytoplasm. Not only does the sperma- 

 tozoon fail to produce activation changes but also the initial 

 changes, produced by the hypertonic treatment, have created 

 an environment in which a spermatozoon very quickly goes to 

 pieces. 



If an egg has not received the optimum exposure for activa- 

 tion, from the hypertonic treatment, a spermatozoon is capable 

 to some extent of provoking activation changes, and a very 

 striking gradient of activity on the part of the egg can be demon- 

 strated from the normal behavior in fertilization to a complete 

 state of responsive inactivity. If the previous treatment has 

 led to segmentation, spermatozoa readily enter the blastomeres, 

 but are entirely incapable of reactivating them. 



And finally, the capacity for fertilization has been found to 

 correspond with the presence or absence of the egg secretion, 

 fertilizin. If the egg possesses any capacity for fertilization 

 after the hypertonic treatment, fertilizin has been found to be 

 present; when the capacity for fertilization is negative, no 

 fertilizin has ever been detected by its sperm agglutinating 



properties. 



II. MATERIAL AND METHODS. 



The material used in this investigation has been confined to 

 the Atlantic sea-urchin (Arbacia punctulatd) found in the Woods 

 Hole region. Fresh material was received daily from the labor- 



