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THE AMERICAN NATURALIST [Vol. XLIX 



Evidence of the physiological individuality of the chro- 

 mosomes may be concluded by referring briefly to the so- 

 called accessory chromosome. This fraction of a chro- 

 mosome, whole chromosome, or in some cases, group of 

 chromosomes, possesses no true synaptic mate, and there- 

 fore at reduction division two types of daughter cells are 

 found. The presence or absence of the "accessory" is so 

 closely associated with sex determination that most biolo- 

 gists now regard it as the morphological expression of a 

 germinal sex determinant. The essential result of re- 

 searches on this body may be summed up in the following 

 words of Wilson. 



They have established the existence of a visible difference between the 

 sexes in respect to these chromosomes, and have shown that it is trace- 

 able to a corresponding difference in the nuclei of the gametes of one 



The simplest type of accessory chromosome, where the 

 male possesses an unpaired chromosome which passes 

 to one pole undivided in one of the spermatocyte divisions 

 and hence enters but half the spermatozoa, was discovered 

 by Henking (1891) in Pyrrhocoris. This work was con- 

 firmed in certain species of Orthoptera in 1902 by Mc- 

 Clung, who advanced the hypothesis that the odd chromo- 

 some was a sex-determiner. Shortly afterward this was 

 made more probable by Wilson and by Stevens who 

 proved for several species of Hemiptera that the body 

 cells of the males contain one less chromosome than the 

 females. Two accessory or X chromosomes are present 

 in the female, while but one is present in the male. 



About the same time, both Wilson and Stevens inde- 

 pendently discovered another kind of dimorphism in male 

 germ cells of certain Hemiptera. Here the X chromo- 

 some of the male has a smaller synaptic mate Y. The 

 body cells of the female, however, show two of the large 

 X chromosomes. The sexes, therefore, both contain the 

 same number of chromosomes, but have the same type of 

 chromatin difference as was first discovered. The female 

 is XX and the male XY. 



