Nature of Formative Stimulation 129 



Two ways were open to find an answer to this question: 

 first to try to cause the development of the unfertiUzed egg 

 with extracts from sperm. I have spent a good deal of time in 

 trying to succeed in this task, but met at first with only negative 

 results for the reason that I used at first only extracts from the 

 sperm of the same species of animals from which the eggs were 

 taken. Only recently have I found that the extract of sperm is 

 effective only if it is taken from a foreign species. We shall 

 return to this curious fact later on and show that it has a 

 bearing upon the problem of the immunity of our cells to the 

 lysins of our body. 



The second way which could lead to a decision of the ques- 

 tion concerning the nature of formative stimulation lay in the 

 direction of artificial parthenogenesis, i.e., of the causation of 

 the development of the animal egg, not by extracts of sperm 

 but directly by physico-chemical agencies. This method of 

 procedure has a special advantage. Since in this case we know 

 the nature of the agencies we employ, it is easier to get an 

 insight into the mechanism by which they cause the development 

 of the egg; while if we work with extracts of sperm we are in 

 the dark as to the chemical character of the active substances. 



II 



We will begin with a description of the method of artificial 

 parthenogenesis in the egg of the Cahfornian sea-urchin, 

 since here this method has been worked out most completely. 

 It may be mentioned that in the eggs of many animals the effect 

 of the entrance of the spermatozoon manifests itself almost 

 instantly by a characteristic change, namely, the formation 

 of the so-called membrane of fertilization. Briefly stated this 

 process may possibly consist in the entrance of sea-water 

 between the surface film and the protoplasm of the egg, where- 

 by the former is lifted up from the protoplasm of the egg and 

 separated from it by a more or less wide, clear space. Figs. 1 



