202 SCIENCE PROGRESS 



number, and, in the second place, each species of animal or 

 plant has a characteristic number. In the division of the 

 cells of the human body, for instance, there are formed thirty-two 

 chromosomes. But to these and many other similar facts no 

 significance could be attached, beyond the obvious one that 

 the nuclear substance is not divided grossly to form a new 

 cell generation, but distributed by a complex and minutely 

 detailed process of subdivision and segregation. 



Not only were the facts of nuclear division without signifi- 

 cance, but the presence of the nucleus itself seemed to be 

 meaningless. The contractility of the muscle cell, the con- 

 ductivity of the nerve cell, the chemical activities of the gland 

 cell, reside in the cell body, and not, save perhaps in the last 

 case, at all in the nucleus. Throughout cell life it lies to all 

 appearance an inert mass, which becomes active only in the 

 process of cell division. And yet actual experiments on 

 enucleated fragments of Protozoon cells and on the nerve 

 cells of higher forms had proved that in the absence of the 

 nucleus the cell body cannot live. True, it carries on all the 

 life functions for a while, but it seems to have lost with 

 the nucleus the power of growth and of repair. 



The last six years have witnessed the rehabilitation of the 

 nucleus, and biologists now see in it the seat of that influence 

 which directs the formative process by which living matter is 

 produced from non-living matter, and controls the distribution 

 of characters in heredity. 



The actual agent in the latter process seems to be the 

 chromosome, and the material basis of the limitation in the 

 number of characters transmitted from generation to generation, 

 and of their definiteness lies in the restriction of the number of 

 chromosomes to a definite number for each species of animal 

 or plant. The chromosomes are not fragments of nuclear sub- 

 stances of accidental composition, nor are they all alike. On 

 the contrary, the probability is that they are unlike, and 

 possessed of a high degree of individuality. 



The material process which underlies the segregation of 

 characters in the germ cell and the fusion of characters in 

 pure and cross-bred zygotes can also be followed in the peculiar 

 features of the cell divisions which form the male and female 

 gametes. 



As I have already said, each species has a characteristic 



