22 (192) Society for Experimental Biology and Medicine. 



one large chromosome (the " accessory ") in the somatic nuclei of 

 the male can only mean that males arise from eggs fertilized by 

 spermatozoa that lack such a chromosome, and that the single 

 "accessory" of the male is derived in fertilization from the egg- 

 nucleus. 



In type B all the eggs must contain a chromosome correspond- 

 ing to the large idiochromosome of the male. Upon fertilization 

 by a spermatozoon containing the large idiochromosome a female 

 is produced, while fertilization by a spermatozoon containing the 

 small one produces a male. 



The two types distinguished above may readily be reduced to 

 one ; for if the small idiochromosome of type B be supposed to 

 disappear, the phenomena become identical with those in type A. 

 There can be little doubt that such has been the actual origin of 

 the latter type, and that the "accessory" chromosome was origin- 

 ally a large idiochromosome, its smaller mate having vanished. 

 The unpaired character of the " accessory " chromosome thus finds 

 a complete explanation, and its behavior loses its apparently 

 anomalous character. 



The foregoing facts irresistibly lead to the conclusion that a 

 causal connection of some kind exists between the chromosomes 

 and the determination of sex ; and at first thought they naturally 

 suggest the conclusion that the diochromosomes and heterotropic 

 chromosomes are actually sex determinants, as was conjectured 

 by McClung in case of the "accessory" chromosome. Analysis 

 will show, however, that great, if not insuperable, difficulties are 

 encountered by any form of the assumption that these chromo- 

 somes are specifically male or female sex determinants. It is 

 more probable, for reasons that will be set forth hereafter, that the 

 difference between eggs and spermatozoa is primarily due to dif- 

 ferences of degree or intensity, rather than of kind, in the activity 

 of the chromosome groups in the two sexes ; and we may here 

 find a clue to a general theory of sex determination that will accord 

 with the facts observed in Hemiptera. A significant fact that 

 bears on this question is that in both types the two sexes differ in 

 respect to the behavior of the idiochromosomes or "accessory" 

 chromosomes during the synaptic and growth periods, these chro- 

 mosomes assuming in the male the form of condensed chromosome 



