THE PHYSICAL BASIS OF LIFE 203 



number of chromosomes, but in the cell divisions which form 

 ova or spermatozoa, ovule or pollen grain, this number is halved, 

 so that each spermatozoon or ovum receives only half the proper 

 number. In this sense, therefore, the germ cell is only half a 

 cell. When two germ cells fuse in the act of fertilisation, the 

 full number of chromosomes is restored. Thus, to choose an 

 instance, the full number of chromosomes which make up a 

 nucleus in a cell of the human body, no matter where it be 

 placed, is 32 ; in the formation of the spermatozoon or ovum, 

 however, there is a redistribution of chromatin, so that each 

 receives only 16 chromosomes. When a spermatozoon fuses 

 with an ovum, a zygote with the full number, 32, is formed. 



The chromosomes therefore are the elements, the organs, as 

 it were, of heredity. They have individuality, the limitations of 

 which are not yet known. Each bears a unit character or a 

 group of unit characters. The evidence for the individuality 

 of the chromosomes is very remarkable. 



Fundulus and Menidia are two fishes belonging to separate 

 orders. Each has 36 chromosomes, but the chromosomes of the 

 former are so much longer than those of the latter as to be 

 readily distinguishable. Moenkhaus 1 crossed these two forms, 

 and traced the fusion of the long and short chromosomes in 

 the formation of the hybrid zygote. But when the zygote pre- 

 pared to divide, the paternal and maternal elements segregated 

 and formed two groups of chromosomes, the one of long and 

 the other of short chromosomes, and in each segment division 

 the paternal long and the maternal short chromosomes re- 

 appeared and acted independently. Another case has been 

 furnished by Wilson. 2 In certain groups of insects there is 

 among the chromosomes of the male cells one distinguished 

 by its small size. The total number of chromosomes, instead 

 of being even, is odd : there are thirteen, and this small 

 chromosome is the thirteenth. It is, in point of fact, only half a 

 chromosome, therefore when each of the others divides into 

 two, it does not divide, but passes bodily to one or other of the 

 two new cells. In this way two different kinds of spermatozoa 

 are formed — those which possess the odd half chromosome, and 

 those in which it is missing — and they are formed in equal num- 

 bers. Now, ova fertilised by the former grow into females, those 



1 Amer.J. Anat. iii. 1904. 



3 Jonrn. of Exp. Zool, iii. 1906. 



