A PHYSIOLOGICAL CHARACTER. 157 



in certain cases, be readily brought about in parthenogenetic reproduction. This is not 

 indicated by the cytological evidence, so far as known, and would seem to call for quite 

 a novel type of cytological behavior for which there is no direct genetic or cytological 

 evidence in parthenogenetic organisms. But should there prove to be such a segregation 

 in Simocephalus, then the effects of selection in this experiment with Cladocera readily 

 yield to explanation in conformity with the chromosome interpretation of the pure-line 

 hypothesis. Otherwise the writer does not see how the results can be satisfactorily made to 

 conform with this theory. The Cladocera material deserves a critical cytological analysis. 

 The writer would gladly cooperate in providing material for such a study by a cytologist 

 who could give ample time to the problem. 



It is just possible, however, that there may be an unusual or at any rate virtually 

 unknown type of chromosomal behavior in Cladocera, permitting segregation more fre- 

 quently than would otherwise be expected in material in which segregation seems improb- 

 able. Wenrich's (1916) observations on Phrynotettix are most suggestive in this connection. 

 He found reduction occurring in a single chromosome and apparently not affecting the 

 other chromosomes in that division. Crossing-over and non-disjunction (which Bridges 

 and others have demonstrated and utilized so successfully in interpreting genetic results 

 with Drosophila and other forms) may be appealed to as affording possible vehicles for such 

 genetic changes, assuming that several genetic factors are involved in the reactiveness to 

 light in Cladocera and that Line 757 was originally heterozygous for certain of these factors. 

 In view of such known irregularities in chromosome behavior, one is led to consider the 

 possibility that a partial segregation may occur in the one maturation division in partheno- 

 genetic reproduction in Cladocera. Nabours (1919) found segregation in parthenogenetic 

 reproduction in one of the grouse locusts, Apotettix. However, judging from the very 

 limited cytological evidence presented, it would seem that in this case reduction was fully 

 accomplished, as in sexual reproduction. 



However, there is no cytological evidence that reduction or segregation of any sort 

 occurs in the one maturation division of parthenogenetic eggs of Cladocera and, even 

 assuming that such a partial segregation may occur in the single maturation division in 

 Cladocera, there is still the theoretical difficulty of accounting for such frequent genetic 

 change as seems to have occurred in Line 757. For it is hard to see how, even with assumed 

 partial reduction in Cladocera (which lack the complicated features of protozoan nuclei), 

 the rate of genetic change should so greatly have exceeded that so carefully worked out 

 for Drosophila. This is not necessarily a fatal objection, however. Even though Cladocera 

 are probably very little subject to genetic change, once genetic change occurs, for any 

 thing we know to the contrary, it may proceed at a rapid rate. 



This possible explanation has a certain appeal to the writer, not only because it offers 

 an explanation (in accord with the explanation offered for most results of selection) for 

 the results attained in the present study of the effects of selection on the basis of reactive- 

 ness to light, but also because certain other (unpublished) results of study of Cladocera 

 material may in part be explained on this basis. 



Explanation of the result with Line 757 by larger mutations 

 encounters considerable difficulty: (1) the necessity for assuming 

 at least three mutations, at least one of which must have been quite 

 large; (2) the lack of definite points at which mutations may be 

 presumed to have occurred; and (3) the occurrence of an additional 

 mutation or mutations effective in the same direction as an earlier 

 one. Further, the mutations in the two strains must have occurred 

 in opposite directions, rendering the plus strain more reactive and 

 the minus strain less reactive. 



In general, in uniparental inheritance mutation may perhaps be 

 expected to be only half as frequent as in biparental inheritance, 

 since in the former a single germ-cell is involved in the production of 

 each individual offspring. Bridges (1919) states evidence tending to 

 show that in Drosophila mutations may occur in oogonial (or sperma- 

 togonial) divisions and in the zygote "immediately after fertiliza- 



