256 GERM-CELL CYCLE IN ANIMALS 
somes. For many years thenumber 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: 
TypeI. One X-chromosome. This, the simplest 
type, has been recently demonstrated in a remarkable 
fashion by Mulsow (1913) in a nematoid worm, 
Ancyracanthus. 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, 4), in female worms, 
twelve (EZ). Two sorts of spermatozoa are produced, 
one-half with five and the other half with six chromo- 
