W. E. Agar 329 



to the theory, over and over again in regard to asexual propagation in 

 plants. The theory has now successfully stood the test in the case of 

 parthenogenetic reproduction, without meiosis, in an animal. 



There is of course no reason to suppose that parthenogenetic repro- 

 duction, as such, must exclude segregation. It is the absence of meiosis 

 which is the crucial point, and we know that in Daphnia and Simo- 

 cephalus, parthenogenetic eggs, whether (/ - or ^ -producing, are matured 

 without reduction of chromosomes (Kuhn, Chambers, Taylor). It is 

 plain that in the parthenogenetic production of haploid eggs from diploid 

 mothers segregation might be expected ; and indeed it appears to have 

 taken place in the stick insects investigated by Fryer. 



In the light of our general knowledge of genetics, and in particular 

 of the experiments just described in Cladocera, we must postpone our 

 interpretation of the segregation with crossing over, which Nabours 

 found in the parthenogenetic reproduction of Apottetix, until the cytology 

 of its oogenesis has been worked out. 



The sexual mode of reproduction is indeed so generally associated 

 with meiosis of the type found in the Metazoa and Metaphyta, and the 

 asexual mode with lack of meiosis, that the words sexual and asexual are 

 often used as if tacitly implying such association. The assumption in- 

 volved by this use of the word must however be kept constantly in mind. 

 Thus, interesting though they are in themselves, and promising of still 

 more interest when properly correlated with cytological observations, the 

 recent experiments on the production by selection of genetic diversity 

 among the asexually produced offspring of certain Protozoa must not, 

 until further knowledge is available, be compared too closely with the 

 production of a similar diversity among Metazoa, in which all the 

 chromatin is concentrated during cell division into chromosomes which 

 are accurately distributed by the mitotic processes. 



It must however be borne in mind that exceptions occur to every 

 rule. Although segregation does not normally occur in the absence of 

 the meiotic processes which precede gametogenesis, it is only to be 

 expected that among the many abnormalities that are known to occur in 

 ordinary somatic mitoses, some may result in irregular distribution of 

 the chromosomes leading to segregation of the contained factors. Cases 

 of somatic segregation have indeed often been described in plants, though 

 in the majority of these there seems to be no valid reason for ascribing 

 the phenomena to segregation rather than to mutation. 



