362 ON THE NUMBER OF POLAR BODIES AND [VI. 



fertilized in the normal female. The question therefore arises 

 as to the way in which the eggs become capable of adapting 

 themselves to the expulsion of two polar bodies when they 

 are to be fertilized, and of one only when fertilization docs not 

 take place. 



But perhaps the solution of this problem is not so difficult as 

 it appears to be. If we may assume that in eggs which are 

 capable of two kinds of development the second polar body is 

 not expelled until the entrance of a spermatozoon has taken 

 place, the explanation of the possibility of parthenogenetic 

 development when fertilization does not occur would be forth- 

 coming. Now we know, from the investigations of O. Hertwig 

 and Fol, that in the eggs of Ediiniis the two polar bodies are 

 even formed in the ovary, and are therefore quite independent 

 of fertilization, but in this and other similar cases a partheno- 

 genetic development of the egg never takes place. There 

 are, however, observations upon other animals which point to 

 the fact that the first only and not the second polar body may 

 be formed before the spermatozoon penetrates into the ^%'g. 

 It can be easily understood why it is that entirely conclusive 

 observations are wanting, for hitherto there has been no reason 

 for any accurate distinction between the first and the second 

 polar body. But in many eggs it appears certain that the 

 second polar body is not expelled until the spermatozoon has 

 penetrated. O. Schultze, the latest observer of the ^g% of the 

 frog, in fact saw the first polar body alone extruded from the 

 unfertilized Qgg : a second nuclear spindle was indeed formed, 

 but the second polar body was not expelled until after fertiliza- 

 tion had taken place. A very obvious theory therefore suggests 

 itself :— that while the formation of the second polar body is 

 purely a phenomenon of maturation in most animal eggs, and 

 is independent of fertilization,— in the eggs of a number of 

 other animals, on the other hand, and especially among Arthro- 

 pods, the formation of the second nuclear spindle is the result 

 of a stimulus due to the entrance of a spermatozoon. If this 

 suggestion be confirmed, we should be able to understand why 

 parthenogenesis occurs in certain classes of animals wherever 

 the external conditions of life render its appearance advan- 

 tageous, and further, why in so many species of insects a 

 sporadic parthenogenesis is observed, viz. the parthenogenetic 



