CHAP. IV 



THE SEX CHROMOSOMES 



99 



term of Sex Chromosomes is, however, a sufficient and obvious description 

 of them, and by this name they will be called here. Owing to the 

 comparative ease and certainty of the observations, and to the interest 

 due to their relation to sex, a great mass of knowledge of these sex chromo- 

 somes has been accumulated in recent years, American cytologists 

 having been particularly active along these lines. 



Fig. 45. 



The chromosomes of Protenor belfragei. (A, I-K, from Morrill, B.B., 1910 ; B-E, H, after Montgomery, 

 Trans. Amer. Phil. Soc, 1901 ; F, G, from Wilson, J.E.Z., 1906.) A, spermatogonia! metaphase group. 

 Twelve ordinary and one sex chromosome. B, prophase I., S ; C, metaphase I., ^ ; D, anaphase I., ^ ; E, 

 early anaphase II. in the ,5. Sex chromosome passing undivided to one pole. F, G, polar views of the 

 chromosome groups at each pole of the spindle in anaphase II. One group with the sex chromosome, the 

 other without. H, late anaphase II., showing one spermatid with the sex chromosome, the other without 

 I, oogonial metaphase group, twelve ordinary and two sex chromosomes ; J, chromosome group from a male 

 embryo ; K, chromosome group from a female embryo. 



X, the sex chromosome. 



As a typical example of an animal with sexual dimorphism of 

 chromosomes we may take the Hemipteran insect Protenor belfragei 

 (Fig. 45). Its chromosome cycle has been worked out by Montgomery 

 (1901 b), Wilson (1906 b, etc.) and Morrill (1910). 



The spermatogonial chromosome groups (A) show thirteen chromo- 

 somes, one being more than twice as large as any of the others. This 



