The Spermatogenesis of Aplopus mayeri. 29 
sory produces a female, the egg must contain the character that determines 
femaleness and the accessory chromosome must be a male sex determinant, 
which, however, is recessive to the dominant female determinant in the egg. 
Extending the above formule (following Wilson) to express these assump- 
tions, they become— 
A. QEgg (18 chrom.) + (¢) Spermatozoon (18 chrom.) = 9 (¢) female (36 chrom.) 
B. ¢ Egg (18 chrom.) + (0) Spermatozoon (17 chrom.) = (¢)(0) male (35 chrom.) 
The facts in Aplopus mayeri admit of interpretation according to Castle’s 
theory of sex-production, and contribute to the cumulative evidence in favor 
of the hypothesis that there exists a causal relation between the accessory 
chromosome and sex-phenomena. 
SUMMARY. 
(a) Primary spermatogonia divide both mitotically and amitotically. In 
the latter instance cell-division is frequently not consummated and a bi or 
multi-nuclear cell results. A binuclear cell has been observed subsequently to 
divide karyokinetically, thus giving rise to primary spermatocytes with 
double the number of chromosomes (2 X 17 + 2 accessory chromosomes = 
36 chromosomes), which may develop into giant spermatozoa. Primary 
spermatogonia have neither accessory chromosome nor plasmosome. 
(b) In the first order of the secondary spermatogonia the accessory 
chromosome appears in the resting-stage. During the late telophase of 
ensuing spermatogonial divisions the accessory is lost (probably assuming 
a brief reticular phase) until a spermatogonium of the last order is attained, 
when the accessory persists as a unique structure characterized by its definite 
form, staining reaction, position in the nucleus, and its behavior during syn- 
apsis and the maturation mitoses. 
(c) During synapsis the accessory chromosome lengthens into a club- 
shaped structure attached by its lesser end to the presynaptic thread, under- 
goes partial longitudinal division, closes up again during the height of 
synapsis, and returns again to its previous characteristic form and location 
in the nucleus of the growing primary spermatocyte. 
(d) Brief resting-stages are interpolated between the telophase of the 
final spermatogonial mitosis and synapsis and between the latter stage and 
the prophase of the first maturation division. The latter resting stage cor- 
responds to a portion of the growth-period of the primary spermatocyte. 
(e) The somatic number of chromosomes for the female Aplopus is 36; 
the spermatogonial number is 35; and the number for the primary spermato- 
cytes is 18. One of these chromosomes is characteristically large U-shaped 
and situated at the periphery of the chromosome complex and is the acces- 
sory chromosome. 
(f) The accessory chromosome passes undivided to one of the poles 
during the primary spermatocyte division. This mitosis is the reductional 
