A THEORY OF FERTILIZATION 49 



including the rabbit, may be stimulated to develop by artificial means. Various 

 agents have been employed. There are some cases — such as in wasps, bees, 

 and ants — where parthenogenetic development is normal. The partheno- 

 genetic eggs usually develop into males, the fertilized ones into females. 



Whatever the mechanism may be, the experiments on artificial partheno- 

 genesis bring out the important fact that all the factors necessary for develop- 

 ment are within the egg, and the sperm contributes no factors for early 

 embryonic development. This means that we must look to the egg for all the 

 substances which are active in differentiation. 



Since eggs will develop as a result of parthenogenetic treatments, it is 

 not surprising that cross-fertilization can be brought about between many 

 animals. The egg, in general, must be considered as a reaction system which 

 is all ready to operate; it needs only some external stimulus. The stimulus 

 is not very specific, since we can substitute chemical agents or the sperm of 

 other species of animals. In cross-fertilization it sometimes happens that the 

 sperm merely stimulates the egg without the sperm chromosomes' com- 

 bining with the chromosomes of the egg. In other cases the chromosomes of 

 the sperm actually combine with those of the egg of a different species, and 

 this sometimes brings about a new kind of individual. Among vertebrates 

 cross-fertilization between different species sometimes results in sterility: the 

 resulting animal is unable to breed. 



A theory of fertilization 



Since the sperm does not add any specific chemical compound to the egg 

 for its early development, almost all theories of fertilization invoke a release 

 mechanism of some sort as an explanation of fertilization. That is, something 

 from the sperm — or, in the case of artificial parthenogenesis, the chemical 

 compounds used — releases a substance already present in the egg. The egg, 

 then, is pictured as having inside it substances in bound, inactive form. The 

 way this situation is usually pictured is that a substance, A, may be sur- 

 rounded by a surface coat of another substance, which we might call B (Fig. 

 22). B forms a shell or barrier around A. If some third compound, C, is 

 present, A and C cannot react until B is removed in some way. 



This release hypothesis often serves as an explanation for various processes 

 during development and for this reason is introduced at this time. All the 



