6 



BIOLOGIC BASIS OF SEX 



some elements and one unpaired element 

 which after spermatocyte divisions led to 

 sperm of two numerically different classes, 

 one carrying 11 chromosomes plus the ac- 

 cessory chromosome and the other carrying 

 only the 11 chromosomes. His observations 

 were confirmed by several other investiga- 

 tors and in principle for other insect species 

 over a decade following. The first sugges- 

 tion that this chromosome behavior was re- 

 lated to sex diiTerentiation came from 

 McClung's (1902) observations on the "ac- 

 cessory chromosome" of Xiphidiiim fascia- 

 tum. It is of some interest to examine the 

 steps l)y which these conclusions were 

 reached. 



"The function exercised by the accessory 

 chromosome is that it is the bearer of those 

 qualities which pertain to the male or- 

 ganisms, primary among which is the fac- 

 ulty of producing sex cells that have the 

 form of spermatozoa. I have been led to 

 this belief by the favorable response which 

 the element makes to the theoretical require- 

 ments conceivably inherent in any struc- 

 ture which might function as a sex determi- 

 nant. 



"These requirements, I should consider, 

 are that: (a) The element should be chro- 

 mosomic in character and subject to the 

 laws governing the action of such struc- 

 tures, (b) Since it is to determine whether 

 the germ cells are to grow into the passive, 

 yolk-laden ova or into the minute motile 

 spermatozoa, it should be present in all the 

 forming cells until they are definitely es- 

 tablished in the cycle of their development, 

 (c) As the sexes exist normally in about 

 (■(lual projoortions, it should be present in 

 half the mature germ cells of the sex that 

 bears it. (d» Such disposition of the element 

 in the two forms of germ cells, paternal and 

 maternal, should be made as to admit of 

 the readiest response to the demands of en- 

 A'ironment i-egarding tlie ])i'oportion of the 

 sexes, (el It should show variations in 

 structure in accordance with the vai-iations 

 of sex potentiality observable in different 

 species, (f) In parthenogenesis its function 

 would be assumed by the elements of a cer- 

 tain polar body. It is conceivable, in this 

 regard, that another form of polar body 

 miglit function as the non-determinant 



bearing germ cell." The important fact es- 

 tablished by this reasoning was that a chro- 

 mosome could be the visual differentiator 

 between the fertilized eggs developing spe- 

 cific adult sexual differences. It w^as of little 

 consequence perhaps that for sex itself, in 

 this species, the chromosome arrangement 

 was misinterpreted. The validity of the 

 rules was probed through examinations of 

 the cells of many species by many different 

 observers. 



B. SEX AS ASSOCIATED WITH VISIBLE 

 CHROMOSOMAL DIFFERENCES 



Variations in the chromosome complexes 

 contributing to sexual differentiation were 

 soon found. Probably the most frequent 

 type observed in animals and plants was 

 that in which a single accessory chromo- 

 some of the male was accompanied by, and 

 paired with, another chromosome either of 

 the same or of a different morphologic type. 

 This other chromosome, as distinguished 

 from the accessory chromosome now gen- 

 erally called the X, was designated the Y 

 chromosome. For different species the Y 

 ranged in size from complete absence, to 

 much smaller, to equal to, to larger than 

 the X. Besides the X and Y chromosomes, 

 the autosomal or A chromosomes, the ho- 

 momorpluc pairs, complete the species 

 chromosome complement. In this terminol- 

 ogy 



Sperm (Y + A) + egg (X + A) 

 = XY + 2A cells 



of 



lie determinino; tvpe 



Sperm (X + A) + egg (X + A ) 

 = 2X + 2A cells 



of female detei'mining tyi)e 



Variations in numbers and sizes of chro- 

 mosomal pairs making up the autosomal 

 sets of different species are familiar cyto- 

 genetic facts. These variations may extend 

 from one pair to many chromosome pairs 

 depending on the species. Similar variations 

 may occur in chromosomes of eitluM- the X 

 or Y types. The X is commonly a single 

 chromosome but may be a compound of as 

 many as 8 chromosomes in Aiicaris inciirva 

 (doodricli. 1916). The Y mav be lacking 



