Studies on Chromosomes 181 



are passing to the other. In this case one pole receives 1 1 chromo- 

 somes, the other 13. It is evident that in this form there is the 

 possibility of forming six classes of spermatozoa, as follows: 



(l) 10 + 7 = II (2) IO + I + 25 = 13 



(3) 10 + 7 + S = 12 (4) 10 + + S= 12 



(5) 10 + / + 2s = 13 (6) 10 + / = ii 



In none of these individuals is the material very favorable for 

 the study of the chromosome-nucleoli. They are always evidently 

 compound, but only in a few cases can the components be clearly 

 recognized (as in Fig. 2, c). 



6 Individuals with twenty-five Chromosomes; three 



Supernumeraries 



No individuals of this type were found in M. femoratus. The 

 other two species are represented by three males and three females 

 but here again the material does not admit of exhaustive study. 

 In one of the females, two of the supernumeraries are large and 

 one small, the ovarian cells showing 25 chromosomes, of which 

 three are very small (Fig. 12, i-k), a condition seen in every group 

 of which a clear view can be had. The two larger supernumer- 

 aries cannot, however, be certainly identified in any of these. In 

 all the other individuals the supernumeraries are of the larger 

 form. Fig. 12, a, b, show the first division in one of these cases 

 (term., No. 34); c-g are spermatogonial groups from the same indi- 

 vidual; h, an ovarian group of the same type. Fig. 12, m-p, are 

 from a doubtful case in which nearly all the first division figures 

 show three supernumeraries (n, o), but a single case (m) shows 

 distinctly four. 



7 Individuals with twenty-seven Chromosomes; five 



Supernumeraries. 



This class is represented by a single very interesting male of 

 granulosus (No. 57), in which only the first division can be satisfac- 

 torily examined. Many polar views of this division show 17 

 chromosomes (Fig. 13, a-/, Photo 10), of which two are always 



