THE CELL-DOCTRINE 131 



chromosomes. These bodies appear at the time of cell 

 division in definite number and, with certain exceptions, in 

 pairs (Fig. 13). The number is as definite a characteristic 

 of a species as five fingers or toes or any other feature which 

 is numerically constant. The pairing is very clear in cases 



*X *x "x *y 



Fig. 16. Chromosomes of Drosophila. Diagram- 

 matic representation of the male and female 

 groups. Note that the chromosomes are in 

 pairs, three of which would be readily distin- 

 guishable if the entire eight chromosomes were 

 irregularly arranged. The pairs marked XX 

 and XY are the sex chromosomes. The hook 

 on the Y is a convention. (Redrawn from a 

 figure by Morgan, et al.). 



where the chromosomes are of different sizes and shapes 

 (Fig. 16). Doubling of the number is prevented, at the time 

 of fertilization, by the fact that in the final stages of the 

 ripening ovum and spermatozoon the number of chromo- 

 somes is reduced one-half. Only one member of each pair of 

 chromosomes goes to a germ-cell. When sperm-nucleus 

 unites with egg-nucleus, the full number of chromosomes is 

 restored and also the pairs. The oosperm is thus like all 

 the later products of its division. When it passes into the 

 two, four, eight, and sixteen cell stages, and so to the adult, 

 the chromosomes divide with every cell division (Fig. 13). 

 Each cell of the adult body possesses a nucleus whose 

 chromosomes have descended, through a long series of cell 

 divisions, from the nucleus of the one cell stage. This 

 originated by a union of equivalent chromosomes from egg 



