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SCIENCE 



[N. S. Vol. XLIII. No. 1111 



delian inheritance have furnished an ex- 

 traordinarily complete demonstration of 

 the existence of such inheritance units and 

 of their persistence generation after gen- 

 eration. Such units are individuals in that 

 they are separate from, though dependent 

 upon, other units and in that they appar- 

 ently manifest the fundamental vital 

 processes of assimilation, growth and di- 

 vision. 



2. The Individuality of Parts of Cells. — 

 Many parts of a cell, such as the chromo- 

 meres, chromosomes, plastids, and in some 

 instances at least, the centrosomes and plas- 

 tosomes are also individuals in this same 

 sense. The question of the individuality of 

 chromosomes and centrosomes has given rise 

 to much controversy chiefly because the term 

 "individual" has not been clearly defined. 

 No one doubts that chromosomes have the 

 power of assimilation, growth and division 

 and the only question at issue is as to whether 

 they disintegrate at the close of every di- 

 vision and are formed anew at the begin- 

 ning of the succeeding division. Now that 

 individual chromosomes have been traced 

 right through the entire resting period in 

 several cases, there is no longer any rea- 

 son to doubt that chromosomes do in some 

 instances preserve their individuality. The 

 fact that they, like all other forms of liv- 

 ing matter, undergo metabolic change, re- 

 ceiving food substances on the one hand, 

 and building them up into their own sub- 

 stance, and on the other hand, giving off 

 the waste products of their own destructive 

 metabolism — in short that the materials of 

 which they are composed are undergoing 

 continual change — does not obscure the in- 

 dividuality of a chromosome any more 

 than a similar process obscures the indi- 

 viduality of a man. That which persists 

 amid all metabolic changes in both the 

 chromosome and the man is not identical 



atoms or molecules, but an identical organi- 

 zation or plan or relation of subordinate 

 parts to one another. 



In my experience the same is true of cen- 

 trosomes; they also undergo growth and 

 division, are continuous from cell genera- 

 tion to cell generation, and do not arise de 

 novo from "cytasters," which are only 

 temporarily isolated portions of archiplasm 

 or kinoplasm, though they are genetically 

 related to achromatic constituents of the 

 nucleus. In all probabilities there are 

 other units in the cell which preserve a 

 like individuality, as, for example, plastids 

 and plastosomes. All such parts of a cell 

 have an individuality of their own, in that 

 they are separate though not independent, 

 and have the properties of assimilation, 

 growth and division. 



3. Individuality of Cells. — The individ- 

 uality of ultra-microscopic units and of 

 visible parts of cells is of a different order 

 from that of entire cells. The former, 

 though separate, are yet so dependent on 

 other units as to be incapable of independ- 

 ent existence. In the cell for the first time 

 we find an organic individual sufficiently 

 independent to carry on by itself all fun- 

 damental processes of life. Protista, germ 

 cells, embryonic cells and tissue cells show 

 this independence in varying degrees, and 

 yet of course, no cell and no higher organ- 

 ism is absolutely independent of other or- 

 ganisms or of the environment. In short, 

 independence is a relative term and is no 

 necessary part of individuality. 



In the union of the egg and sperm cells 

 in fertilization, the cells lose their inde- 

 pendence as cells, though the separateness 

 of parts of these cells may persist. There 

 is here the merging of two cell individual- 

 ities into one, just as in the reverse process 

 of cell division there is the merging of one 

 cell individuality into two. But so far as 



