l8 STUDIES IN SPERMATOGENESIS. 



not possibly be conceived of as a sex determinant. In Sagitta it is 

 known to be confined to the male germ-cells. No such element 

 occurs in the ovogenesis, in the sperm nucleus in the egg, or in the 

 first segmentation spindle. Its function must, therefore, be confined 

 to the process of spermatogenesis. 



From the standpoint of sex determination, we have in Tenebrio moli- 

 tor the most interesting of the forms considered in this paper. In both 

 somatic and germ cells of the two sexes there is a difference not in the 

 number of chromatin elements, but in the size of one, which is very 

 small in the male and of the same size as the other 19 in the female. 

 The egg nuclei of the female must be alike so far as number and size 

 of chromosomes are concerned, while it is absolutely certain that the 

 spermatids are of two equal classes as to chromatin content of the 

 nucleus one-half of them have 9 large chromosomes and i small one, 

 while the other half have 10 large ones. Since the male somatic cells 

 have 19 large and i small chromosome, while the female somatic cells 

 have 20 large ones, it seems certain that an egg fertilized by a sperm- 

 atozoon which contains the small chromosome must produce a male, 

 while one fertilized by a spermatozoon containing 10 chromosomes of 

 equal size must produce a female. The small chromosome itself may 

 not be a sex determinant, but the conditions in Tenebrio indicate that 

 sex may in some cases be determined by a difference in the amount or 

 quality of the chromatin in different spermatozoa. This is much the 

 most suggestive part of the work, and it will be followed up by the 

 study of related forms. 



There appears to be so little uniformity as to the presence of the 

 heterochromosomes, even in insects, and in their behavior when pres- 

 ent, that further discussion of their probable function must be deferred 

 until the spermatogenesis of many more forms has been carefully 

 worked out. 



BRYN MAWR COU^GE, May 75, 1905. 



