256 GERM-CELL CYCLE IN ANIMALS 



somes . For many years the number of chromosomes in 

 the cells of the individuals of a species was considered 

 constant and even. Henking, however, in 1891, 

 discovered in the bug, Pyrrhocoris, a single chro- 

 mosome which did not divide in one of the sperma- 

 tocyte divisions, but passed to one of the daughter 

 cells and hence into only one-half of the spermatozoa. 

 Paulmier (1899) observed similar conditions in 

 the squash bug, and since then one or more odd 

 chromosomes have been discovered in a large number 

 of animals belonging to many different phyla. In 

 1902, McClung suggested that these peculiar chro- 

 mosomes might be sex-determinants, and subsequent 

 discoveries have fully demonstrated that they are 

 intimately associated with the phenomena of sex. 

 Most of our knowledge of this subject is due to the 

 investigations of cytologists in this country, es- 

 pecially Montgomery (1898, 1906, 1911), McClung 

 (1899, 1902, 1905), Stevens (1905, 1906, 1910), 

 Wilson (1905, 1906, 1911, 1912), and Morgan (1909, 

 1911, 1913, 1914). A few of the principal types of 

 sex-chromosome distribution are as follows : 



Type I. One X-chromosome. This, the simplest 

 type, has been recently demonstrated in a remarkable 

 fashion by Mulsow (1913) in a nematoid worm, 

 Ancyr acanthus. Here the chromosomes can be seen 

 not only in stained material but also in the living 

 germ cells. The diploid number of chromosomes in 

 male worms is eleven (Fig. 64, A), in female worms, 

 twelve (E) . Two sorts of spermatozoa are produced, 

 one-half with five and the other half with six chromo- 



