136 ALICE M. BORING. 



the first. Fig. 18 shows the two chromosome groups from the 

 ends of a spindle. Each chromosome is splitting, preparatory 

 to the second division, as is also the odd chromosome. The two 

 groups were not in the same focus, and it was clear that the 

 odd chromosome belonged to the lower group. The size relations 

 here are as before, one largest chromosome (A) in each group, 

 and two almost as large (B, C). The odd chromosome (X) 

 here appears slightly larger than the smallest, but they are 

 rearranging themselves in shape and position for the second 

 division, and therefore we cannot be sure that we are focusing 

 on similar sides. 



The equatorial plates in the spermatogonia and in the follicle 

 cells are interesting to compare with those in the spermatocytes, 

 for in both of the former, the unreduced number of chromosomes 

 is present. The grouping in the spermatogonia is more compact 

 than in the somatic cells (Figs. 19, 20), but in both there are two 

 longest chromosomes (A A) and four almost as long (BB, CC), 

 the three pairs represented by the one largest and two almost 

 as large of the spermatocytes. In both also there is an odd 

 number of chromosomes, 23, the condition invariably found in 

 the spermatogenesis of forms with an odd chromosome. Which 

 of the smaller chromosomes here is the odd one, there is no way to 

 detect. In one somatic cell where the chromosomes are much 

 longer and thinner than in any others (Fig. 22), the size relations 

 are not so clear: there are two distinctly longest (A A), but 

 apparently six next longest. However, the chromosomes are so 

 long and twisted in this case that there is every possibility of 

 having misrepresented their lengths in a flat drawing. 



The equatorial plates of the oogonia and female somatic cells 

 are similar to the corresponding male cells, except for the posses- 

 sion of one more chromosome, 24 instead of 23 (Figs. 23-25). 

 This additional chromosome must be a small one, for there are 

 only two longest (A A) chromosomes and four almost as long 

 (BB, CC), as in the male cells. Presumably then this extra 

 chromosome is the mate of the small odd chromosome of the male. 

 Therefore, Philanus spumarius has dimorphic spermatozoa, and 

 one more chromosome in all female cells than in the male cells, 

 one usual expression of sex differentiation in insects. 



