g^ - CYTOLOGY CHAP. 



Duesberg, 1908) the first division is abortive as in the bee, but the 

 second is normal, resulting in two equal and similar spermatids. 



(3) The Homology of the Meiotic Divisions in Obligatory and Facultative 



Parthenogenesis 



The above account of the spermatogenesis of the bee shows that 

 it is the first division of the meiotic phase which is omitted, -and this 

 is in accordance with the consensus of recent opinion that it is this 

 division which usually brings about the segregation of the homologous 

 chromosomes. It would be of great interest to determine which of the 

 two divisions it is which is omitted in the maturation of eggs preparing 

 for obligatory parthenogenesis, if indeed the single maturation division 

 of these eggs can be homologized with either of the divisions of an 

 ordinary sexual meiosis. It used generally to be assumed that it is the 

 second division which is omitted, but there is Httle evidence either way. 

 What there is, is perhaps in favour of this view. Weismann and 

 Ishikawa (1887) found that the single polar body produced by the 

 parthenogenetic eggs of Cladocera frequently divides into two or more 



cells a feature perhaps more characteristic of the first than of the 



second polar body of an ordinary oogenesis. Again, Brauer's observation 

 that in Artemia, after the first polar body has been formed in the usual 

 way, a more or less abortive attempt is in rare cases made to form another, 

 probably indicates that the single maturation division of the majority 

 of eggs corresponds to the first one of a sexual meiosis. As, however, the 

 second division is still unaccompanied by reduction of chromosome 

 number (Hke the two divisions without reduction inNematus and Rhodites'^) 

 it is possibly useless to attempt to homologize the two divisions with the 

 first and second divisions respectively of an ordinary meiosis. 



Even though it were shown that the single division of obhgatory 

 parthenogenesis has all the characteristics (except that of halving the 

 chromosome number) of the normal first meiotic division, this need 

 cause Httle surprise. For few things are better established in cytology 

 than the fact that in the case of one type of chromosome— the sex 

 chromosomes (Chapter IV.)— the first is the reduction division in some 

 animals and the second in others, even nearly allied species differing in 

 this respect. 



(4) Artificial Parthenogenesis 



It has long been known that eggs which normally only develop after 

 fertilization may be induced by certain agents to develop without fusion 



1 It is perhaps significant that there is found a variation in the chromosome number both 

 in Nematus and Rhodites somewhat similar to that in the bee (p. 91)- 



