The Spermatogenesis of Aplopus mayeri. 24 
an equatorial plate it is usually recognizable by its larger size or U-shaped 
form. The accessory chromosome, once having appeared in the spermato- 
gonial cell, preserves its identity and morphological individuality unimpaired 
until it disappears in the ripening spermatozoon ; and though it is never seen 
isolated within a separate vesicle, as described by Sutton (1900) for 
Brachystola magna (while each pair of spermatogonial chromosomes also 
become inclosed in a separate compartment of the nucleus) and Baumgartner 
“(1904) for Gryllus domesticus, it nevertheless doubtless preserves also a 
strict physiological individuality, since it never unites with the ordinary 
chromosomes (except when connected for a brief period with the presynap- 
tic thread) and ultimately passes to one-half of the spermatozoa, thus prob- 
ably altering the physiological activity of those possessing it as compared 
with those lacking it in providing for the former a sex-determining factor. 
DETERMINATION OF SEX. 
In Aplopus a dimorphism of spermatozoa has been demonstrated, con- 
sisting in the presence of an accessory chromosome in one-half of the sperm- 
atozoa and its absence in the other half. McClung (1900) first suggested 
the possible causal connection between the dimorphism of sex and the ob- 
served dimorphism of spermatozoa. His conclusions were drawn from 
observations on some of the insects and the fact that sex appears to be the 
one character that divides the individuals of a species into two approximately 
equal groups. 
Sections of ovarian material presented several favorable opportunities 
for making chromosome counts in somatic mitoses in the female. Equatorial 
plates of follicular cells of the developing ovum in mitosis yielded a chromo- 
some complex, very distinct, and well separated. Though the number of 
such plates was not as large as could have been desired for absolute cer- 
tainty, I am convinced that the somatic number of chromosomes in the 
female is 36 (fig. 19). Any number of very favorable equatorial plates of 
spermatogonial cells in mitosis give a chromosome count of 35 (fig. 17). 
Figure 114 and others give a clear demonstration of a dimorphism of sec- 
ondary spermatocytes consisting in the presence of a large U-shaped chro- 
mosome in one-half the cells when the number of chromosomes is 18. Cells 
lacking this odd element have a chromosome count of only 17. 
The reduced number of chromosomes in the mature egg must be 18, and 
an egg fertilized by one or the other type of spermatozoon will develop 
into an organism with 36 or 35 somatic chromosomes. Obviously the for- 
mer (female) contains the accessory chromosome, and the latter (male) 
lacks it. From the chromosome standpoint the presence of an additional 
chromosome (the accessory chromosome) distinguishes the female cell from 
the male; hence the accessory chromosome appears to have some connection 
with the sex the organism is to acquire. 
