Maech 8, 1907] 



SCIENCE 



377 



tion has been decided it will be hazardous 

 to venture any conclusions as to the causal 

 relation between sex and fertilization. "We 

 may, however, be quite certain that in 

 parthenogenesis the egg alone is competent 

 to determine sex; and for this reason one 

 finds it difficult to avoid the feeling that 

 this is likely to be true of eggs that are 

 fertilized, even though we have direct evi- 

 dence of this in only two or three cases. 

 There is a certain amount of cytological 

 evidence that such is actually the case, at 

 least in the Hemiptera and Coleoptera, and 

 probably in other forms that possess a pair 

 of idiochromosomes or an odd chromosome. 

 On the other hand, the strongest piece of 

 evidence against this is, of course the long- 

 standing one of the bee; yet even here one 

 of several possibilities is that only those 

 eggs are fertilized that are already pre- 

 destined as females. This case, I believe, 

 is not as yet closed. 



For the foregoing reasons it seems to 

 me that our best hope of a successful at- 

 tack on the problem lies in the study of 

 parthenogenesis; though we are here still 

 confronted by too complicated and puzzling 

 an array of facts to be at present sur- 

 mounted by any one interpretation. The 

 limitation of time forbids any adequate 

 review of these facts, and I must limit my- 

 self to a single and, I fear, somewhat one- 

 sided line of treatment. It seems to me 

 that the most available stepping-stone to- 

 wards the investigation of this problem is 

 afforded by recently acquired evidence that 

 sex production stands in some definite 

 causal relation with the chromosomes and 

 may be treated from the standpoint of the 

 Mendelian phenomena, as interpreted by 

 the Sutton-Boveri chromosome theory. It 

 is certain that in many of the insects there 

 is a particular pair of chromosomes that 

 have a special and constant relation to sex 

 production. There is perfectly clear evi- 

 dence that the two members of this pair 



couple in synapsis and are disjoined in the 

 reducing division. There is very strong, 

 though indirect, evidence that one of them 

 enters a male-producing spermatozoon, the 

 other a female-producing one. A very 

 definite material basis, therefore, exists for 

 a treatment of the sex characters as if they 

 were Mendelian alternates, sex determina- 

 tion, as opposed to sex heredity, being a 

 matter of Mendelian dominance, more spe- 

 cifically of chromosome-dominance. I 

 think that apart from the specific evidence 

 in favor of this view a strong a priori argu- 

 ment in its favor is the approximate nu- 

 merical equality of the sexes, which may 

 be taken as the prevailing rule. The exist- 

 ence of a pair of chromosomes that are 

 specifically related to sex production, and 

 in respect to which the gametes of both 

 sexes fall into two equal classes, gives a 

 simple and natural basis for an equal pro- 

 duction of males and females if we assume 

 that these chromosomes embody respect- 

 ively the male-producing and the female- 

 producing factors. In other words, these 

 two chromosomes may represent the hered- 

 itary bases of the male and female charac- 

 ters, respectively; and their relations to 

 each other in respect to dominance may 

 condition sex determination as opposed to 

 sex heredity. 



Let us briefly consider sex determination 

 in parthenogenesis from this point of view. 

 In the parthenogenetie egg sex might con- 

 ceivably be determined either by elimina- 

 tion from the egg of the male or female 

 element in maturation, or by conditions 

 that affect the relations of dominance be- 

 tween the chromosomes. The hypothesis 

 of elimination, which has been discussed 

 especially by Castle and Doncaster, de- 

 mands a reducing division, at least in case 

 of the sex chromosomes; and such a divi- 

 sion is not known to occur without at least 

 a temporary reduction in the number of 

 the chromosomes. This view may perhaps 



