30 HOW ANIMALS DEVELOP 



appear before the germ-cell division they are not 

 split longitudinally (Fig. 3). They do not seem to 

 be able to proceed with the division until they have 

 arranged themselves into double bodies, to corre- 

 spond with the two half-chromosomes lying side by 

 side which are found in ordinary division. They get 

 into a doubled condition in the only way which is 

 open to them ; that is, by the two whole chromosomes 

 belonging to a pair joining up with each other and 

 lying side by side. If there are six chromosomes, 

 two A's, two B's, and two C's, for example, the two 

 A's always join up, and so do the two B*s and the 

 two C's. The chromosomes are now a series of 

 paired bodies, which are just like the split chromo- 

 somes of an ordinary cell-division to look at, except 

 that there are only half as many of them. They go 

 on behaving just like the split chromosomes described 

 above ; that is to say, the two partners in each paired 

 body, which have only just come together, now 

 proceed to separate, one partner going into each of 

 the two daughter-cells. This means that the two 

 daughter-cells have only got half the normal number 

 of chromosomes and are an exception to the general 

 rule in that they have only one chromosome of each 

 kind. That is one of the most important character- 

 istics of the germ-cells. The ordinary body-cells, 

 which all have two of each kind of chromosome, are 

 said to have the diploid number, and the germ-cells, 

 which have only one of each kind, are said to have 

 the haploid number. The process which has just been 

 described is spoken of as the reduction division of the 



