578 SCIENCE PROGRESS 



male-producing and female-producing is indicated by a chain 

 of evidence so strong and detailed as apparently to leave no 

 escape from the conclusion. The crucial evidence is given by 

 a comparison of the diploid chromosome-groups of adults and 

 embryos of the two sexes. These groups are found to show 

 certain characteristic and constant differences which are pre- 

 cisely such as would result from fertilisation of the eggs by the 

 two respective classes of spermatozoa. In all forms such as 

 Pyrrhocoris or Protenor, which have a single X-chromosome 

 in the male, the female diploid groups differ from the male in 

 the presence of one additional chromosome (as was first shown 

 by the writer in certain Hemiptera). Further, the additional 

 chromosome in the female is a duplicate of the unpaired 

 X-chromosome of the male, as is particularly well shown in 

 Pyrrhocoris or Protenor, where the X-chromosome is at once 

 recognisable by its size in both sexes. The female groups are 

 otherwise identical with those of the male (figs, id, e\ fig. 2). 

 These results, first reached in the case of dividing cells of the 

 testis and ovary, have now been extended to cells from other 

 organs and tissues, and to embryonic stages (Morgan, Stevens, 

 Morrill), so that the sex of the embryo may at once be dis- 

 tinguished, even in the earliest stages of cleavage, by mere 

 inspection of the chromosome-groups. 1 



Such a diploid group as that seen in the female gives rise, 

 upon reduction, to a haploid group that contains but one 

 X-chromosome (fig. 2) ; and each mature egg, as well as each 

 polar body, contains such a group. That reduction in the 

 female should produce such a haploid group is evident from 

 the composition of the diploid groups, and the fact has now 

 been directly observed by Morrill 2 in certain insects (Anasa, 

 Protenor, Archimerus, Chelinidea), and by Boveri and Gulick 3 

 in the nematode Heterakis, which possesses a typical single 

 " accessory " or " X-chromosome " in the male. Reduction in 

 the male, on the other hand, results in the presence of the 

 X-element in but half the spermatozoa, while half are without 

 it. In Heterakis, for example, all the mature eggs receive 

 five chromosomes, while half the spermatozoa receive five and 

 half but four. A moment's consideration shows that the 

 characteristic diploid combination of the male will arise upon 

 fertilisation of the egg by a spermatozoon of the Y-class 

 1 Morrill, Science, December 31, 1909. ■ Op. cit. 3 Op. cit. 



