6 Edmund B. Wilson 



In order to give a wider basis of comparison I have given new 

 figures of the chromosome-groups of nearly all the species, 

 even in the case of forms already figured in my preceding papers. 

 Since the idiochromosomes or the heterotropic chromosome form 

 the distinctive differential between the nuclei of the two sexes, I 

 shall in the descriptive part of this paper call them the "differen- 

 tial chromosomes." 



B. First Type. Forms Possessing an "Accessory" or Hetero- 

 tropic Chromosome 



As stated above, I have compared the males and females in 

 respect to the chromosome-groups in four genera, selecting for 

 this purpose the most available cells, which are the dividing 

 oogonia and ovarian follicle-cells in the female, the spermato- 

 gonia and investing cells of the testis-cysts in the male. The 

 general result is the same in all, but owing to the conspicuous 

 size-difference of the chromosomes in Protenor, this form gives 

 the most obvious and striking evidence. 1 



a. Protenor belfragei 



Montgomery ('01) first made known the general character of 

 the chromosome-groups in this interesting species, showing that 

 the spermatogonial groups show an odd number, thirteen, that 

 the heterotropic chromosome (Montgomery's "chromosome #") 

 is immediately recognizable by its enormous size it is fully twice 

 the size of the largest of the other chromosomes and that it is 

 unpaired (though he considered it a bivalent). My own observa- 

 tion confirms his description in every point, except that I have 

 never seen this chromosome transversely constricted into two 

 halves. The first glance at a good preparation of the spermat- 

 ogonial metaphase, as seen in polar view, shows this huge chro- 



^here can be no doubt of the identification of the follicle-cells; but there is some uncertainty regard- 

 ing the cells here called oogonia, since they are from the undifferentiated region of the ovary in which the 

 distinction between oogonia and follicle-cells cannot be made out. It is therefore quite possible that some 

 of the groups here described as oogonia may be from very young follicle-cells or nutritive cells; but this 

 does not affect the main result. 



