FERTILIZATION 203 



plete this mechanism and enable development to proceed. In 

 its other form this idea is that the ovum is a quiescent, passive 

 body which needs to be stimulated to its normal activity 

 (development) by the entrance of the spermatozoon, the kino- 

 plasm (centrosome) of which is the part chiefly acting as the 

 stimulus. In short, fertilization is to be regarded normally as 

 the necessary antecedent, as the cause of development. 



There are many facts opposed to this view. Of these we shall 

 discuss two chief classes, first, those of parthenogenesis, both 

 normal and "artificial," and second, those drawn from the 

 relation between fertilization and reproduction among the 

 Protozoa. 



For present purposes we may extend the definition of partheno- 

 genesis to include all those cases where single cells, specialized 

 for the purpose, develop without undergoing syngamy. In 

 the plant kingdom parthenogenesis, in this broad sense, is very 

 widespread. Development from spores is very common, even 

 among the higher (vascular) plants, and in some instances 

 (some of the Fungi) reproduction by single unfertilized cells is 

 the exclusive method. And the development of ova, typical 

 in every respect save that of needing to be fertilized, is not 

 uncommon. Among the single-celled animals phenomena 

 equivalent to development from spores are frequent, and among 

 the multicellular animals normal parthenogenesis is known in 

 the Rotifera, some of the Crustacea, and in several orders of 

 Insecta. In most of these forms fertilization does occur at 

 some period in the life cycle, after a widely variable number 

 of parthenogenetically produced generations, but there are a 

 few Metazoa, e.g., the wasp, Rhoditis, and the Crustacea, Cypris, 

 Limnadia, and sometimes Apus, in which males never develop 

 and fertilization is therefore entirely unknown (Weismann). 

 In some of the parthenogenetic Crustacea the nucleus of one 

 of the polar bodies seems to act as a fertilizing nucleus (see 

 Chapter IV), so that a sort of autogamic process occurs, 

 recalling that of some of the Protozoa and perhaps analogous 

 with it. In all of these Metazoa the form and history of the 

 parthenogenetic ova, the occasional presence of vestigial 



