October 20, 1905.] 



SCIENCE. 



601 



I 



all of the eggs receive the same number of 

 chromosomes. This number (eleven in Anasa, 

 seven in Proteno?- or AlydusY is the same as 

 that present in those spermatozoa that contain 

 the ' accessory ' chromosome. It is evident 

 that both forms of spermatozoa are functional, 

 and that in type A females are produced from 

 eggs fertilized by spermatozoa that contain 

 the ' accessory ' chromosome, while males are 

 produced from eggs fertilized by spermatozoa 

 that lack this chromosome (the reverse of the 

 conjecture made by McClung). Thus if n 

 be the somatic number in the female n/2 is 

 the number in all of the matured eggs, n/2 

 the number in one half of the spermatozoa 

 (namely, those that contain the 'accessory')? 

 and n/2 — 1 the number in the other half. 

 Accordingly : 



In fertilization 



Egg — + spermatozoon -- = n( female). 



Egg — -f- spermatozoon — ■ 



l^n — 1 (male) 



The validity of this interpretation is com- 

 pletely established by the case of Protenor, 

 where, as was first shown by Montgomery, the 

 ' accessory ' is at every period unmistakably 

 recognizable by its great size. The spermato- 

 gonial divisions invariably show but one such 

 large chromosome, while an equal pair of 

 exactly similar chromosomes appear in the 

 oogonial divisions. One of these in the fe- 

 male must have been derived in fertilization 

 from the egg-nucleus, the other (obviously 

 the ' accessory ') from the sperm-nucleus. It 

 is evident, therefore, that all of the matured 

 eggs must before fertilization contain a chro- 

 mosome that is the maternal mate of the 

 ' accessory ' of the male, and that females are 

 produced from eggs fertilized by spermatozoa 

 that contain a similar group {i. e., those con- 

 taining the ' accessory '). The presence of but 

 one large chromosome (the ' accessory ') in 

 the somatic nuclei of the male can only mean 

 that males arise from eggs fertilized by sper- 

 matozoa that lack siich a chromosome, and 

 that the single ' accessory ' of the male is de- 

 rived in fertilization from the egg nucleus. 



In type B all of the eggs must contain a 

 chromosome corresponding to the large idio- 



chromosome of the male. Upon fertilization 

 by a spermatozoon containing the large idio- 

 chromosome a female is produced, while fer- 

 tilization by a spermatozoon containing the 

 small one produces a male. 



The two tyj)es 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 ' chro- 

 mosome was originally a large idiochromo- 

 some, its smaller mate having vanished. The 

 unpaired character of the ' accessory ' chromo- 

 some 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 

 idiochromosomes and , heterotropic chromo- 

 somes are actually sex determinants, as was 

 conjectured by McGlung in case of the ' acces- 

 sory ' chromosome. Analysis will show, how- 

 ever, that great, if not insuperable, difficulties 

 are encountered by any form of the assump- 

 tion that these chromosomes 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 differences 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 deterrainatioji 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 idiochromo- 

 somes or ' accessory ' chromosomes during the 

 sjaiaptic and growth periods, these chromo- 

 somes assuming in the male the form of con- 

 densed chromosome nucleoli, while in the fe- 

 male they remain, like the other chromosomes, 

 in a diffused condition. This indicates that 

 during these periods these chromosomes play 

 a more active part in the metabolism of the 



