TENEBRIO MOLITOR. 13 



tubes were sectioned and the chromosomes counted in the dividing 

 cells of the egg-follicle ( <? somatic cells), and in dividing oogonia. 

 In both cases 20 large chromosomes were found. Figure 207 is the 

 equatorial plate from a female somatic cell of a young egg-follicle. 

 Figure 208 a and b shows two sections of an oogonium in the prophase 

 of mitosis. In order to determine the number and character of the 

 chromosomes in the male somatic cells, several male pupae were sec- 

 tioned. As in the spermatogonia, 19 large chromosomes and i small 

 one were found. Figure 204 shows the equatorial plate of a dividing 

 male somatic cell, and figures 205 to 206 are daughter plates from a 

 similar cell. (Three large chromosomes of the plate shown in figure 

 206 are in another section.) 



From these facts it appears that the egg-pronucleus must in all 

 cases contain 10 large chromosomes, while the spermatozoon in fertili- 

 zation brings into the egg either 10 large ones or 9 large ones and i 

 small one. Since the somatic cells of the female contain 20 large 

 chromosomes, while those of the male contain 19 large ones and i 

 small one, this seems to be a clear case of sex-determination, not by 

 an accessory chromosome, but by a definite difference in the character 

 of the elements of one pair of chromosomes of the spermatocytes of the 

 first order, the spermatozoa which contain the small chromosome 

 determining the male sex, while those that contain 10 chromosomes 

 of equal size determine the female sex. This result suggests that there 

 may be in many cases some intrinsic difference affecting sex, in the 

 character of the chromatin of one-half of the spermatozoa, though it 

 may not usually be indicated by such an external difference in form or 

 size of the chromosomes as in Tenebrio. It is important that related 

 forms should be studied in order to ascertain whether the same chro- 

 matic conditions prevail in other species of this genus or possibly in 

 the Coleoptera in general.* 



Aphis oenotherae. 



The spermatogensis of Aphis has been fully described in another 

 paper and will merely be briefly summarized here for the purpose of 

 comparison with other forms. 



The spermatogonia contain a large nucleolus, which gradually 

 disappears in the prophases of mitosis (plate vn, figs. 209-211). The 

 youngest spermatocytes closely resemble the spermatogonia (fig. 212). 

 There is no bouquet stage and no such marked spireme stage as in 



* Prof. E. B. Wilson has recently found a similar dimorphism in the spermatozoa of 

 Lygceus and other of the Hemiftera heteroftera. 



