TRIRHABDA VIRGATA. 35 



Sections of male pupae were examined for equatorial plates of 

 somatic mitoses. Figure i is a specimen of sucli plates. As might 

 be expected, this figure resembles quite closely the spermatogonial 

 equatorial plate (fig. 3) in number, form, and size of chromosomes, the 

 small one being present in both. Figure 2 is from the follicle of a 

 young egg ; here we find 28 chromosomes, but no small one. The 

 chromosome corresponding to the larger member of the unequal pair 

 in the male evidently has a homologue of equal size in the female. The 

 chromosome relations in the male and female somatic cells are there- 

 fore the same as in Tenebrio molitor, and must have been brought about 

 by the development of a male from an &%% fertilized by a spermatozoon 

 containing the small chromosome, and a female from an &%% fertilized 

 by a spermatozoon containing the larger heterochromosome, 



Trirhabda canadense. 



In Trirhabda canadense the spermatogonial chromosomes are invaria- 

 bly smaller than in T. virgata, but similar size relations prevail. The 

 Spermatogonial plate (fig. 21) contains 30 chromosomes, 29 large and 

 I extremely small. In the growth stages the association of the two 

 unequally paired chromosomes with a rather large plasmosome is more 

 evident than in T. virgata (figs. 22-23). In this species the unequal 

 pair is more often found at a different level from the other chromo- 

 somes in the early metaphase of the first maturation mitosis (fig. 24), 

 but it later comes into the plate with the other chromosomes (figs. 

 25-27), and divides earlier than most of the other bivalents (fig. 27). 

 In a polar view of this metaphase the largest chromosome often appears 

 double (fig. 28) ; in a front view it is a tetrad as in T^ virgata, figure 

 10. Figure 29 is the equatorial plate of a metaphase in which the 

 larger component of the unequal pair has been removed in sectioning. 

 The daughter plates of a first spermatocyte in anaphase (fig. 30) show 

 the separation of the components of the heterochromosome pair ; and 

 equatorial plates of the resulting two classes of second spermato- 

 cytes (fig. 31) show the same conditions. Figures 32 and 33 are 

 prophases of the second division, figure 33 showing the small chromo- 

 some ready for metakinesis. It was impossible here also to get good 

 drawings of daughter plates of the second spermatocytes to show the 

 content of the two classes of spermatozoa, but there is no doubt that 

 all of the chromosomes divide in the second mitosis, giving one class 

 of spermatids containing the small chromosome, the other class its 

 larger homologue. 



No male somatic cells were found in mitosis, but they would, if 

 found, show the same conditions as in the spermatogonia. One of 



