THEORIES AND PROBLEMS OF CANCER 68 1 



cell. New nuclei are formed from the chromosomes collected 

 together at what were the poles of the spindle ; when division 

 takes place in the daughter cells, precisely the same process is 

 gone through again. 



Now it has already been pointed out that the fertilised ovum 

 is produced by the fusion of two cells, generally from two indi- 

 viduals of opposite sexes. How then are we to account for the 

 fact that the number of chromosomes is not doubled upon the 

 production of a new individual? Some time before those cells 

 are produced which are actually separated from the body in the 

 form of sexual cells or gametes, a mode of division takes place 

 which differs in important respects from the ordinary or 

 somatic l mode of division ; in this form, which has been called 

 " meiotic " division, 2 only half the number of chromosomes 

 usually found in the cells of the species appears. Actually 



CL 



C 







n 



Sj 



each of the chromosomes represents two of the chromosomes 

 which appear in the ordinary or somatic form of division. 

 Instead of appearing in the form of rods, U's or V's, the 

 chromosomes appear in certain peculiar forms constant in 

 character in each species. In species that have been particu- 

 larly investigated a definite number of shapes and a definite 

 number of each of these shapes is found in each cell as it passes 

 through the meiotic phase. 3 The shapes and the number of 

 chromosomes of each shape in man are indicated in the accom- 

 panying figure. When these chromosomes become attached to 

 the spindle fibres, they lie with their long axes parallel to the 

 spindle, that is to say, at right angles to the position taken up by 

 the chromosomes in a somatic division. When they divide, 



1 Appertaining to the soma or body in contradistinction to the reproductive 

 (gametogenic) cells. 



3 Farmer and Moore, Quart. Journ. Micro. Science, vol. xlviii. Part 4, 1905. 



3 Baumgartner, Biol. Bull. 1904, vol. viii. p. 1. Moore and Arnold, Proc. 

 Roy. Soc. London, 1906, vol. lxxvii. B. p. 563. The Meiotic Phase includes 

 the whole series of phenomena in which the chromosomes are reduced in 

 number to one-half of that found in the somatic (body) cells. 



