1906.] on The Plmjsical Basis of Life. 413 



but the presence of the niicleiis itself seemed to be meaningless. 

 The contractility of the muscle cell, the conductivity of the nerve 

 cell, tlie chemical activities of the gland cell, reside in the cell body, 

 and not, save perhaps in tlie last case, at all in the nucleus. Through- 

 out 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 chromo- 

 some, 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 chromo- 

 somes are not fragments of nuclear substances of accidental composi- 

 tion, 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 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, there- 

 fore, 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, 

 tliere is a redistribution of chromatin, so that each receives only IG 

 chromosomes. When a spermatozoon fuses with an ovum, a zygote 

 with the full munber, 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. 



Fandidus and Menidia are two fishes belonging to separate orders. 

 Each has 3G chromosomes, but the chromosomes of the former are so 

 much longer than those of the latter as to be readily distinguishable. 



