April 14, 1916] 



SCIENCE 



525 



separateness and independence are con- 

 cerned, the fertilized egg cell or oosperm, 

 and the fully formed organism into which 

 it develops are one and the same individ- 

 ual, though differing greatly in complex- 

 ity. This fertilized egg fuses with no other 

 cells, it takes into itself no ready-made liv- 

 ing substance, but manufactures its own 

 protoplasm from food substances; it car- 

 ries on its own processes of assimilation, 

 growth and division — in short it is a sepa- 

 rate and highly independent living thing 

 which may be designated as an organism. 



The complexity of any individual is pro- 

 portional to the number of unlike units 

 which constitute it, and this is as true of 

 a chromosome as it is of a person. A com- 

 mon mistake is the supposition that com- 

 plexity is determined by the number of 

 cells, whether like or unlike, composing a 

 body. On the other hand, as Whitman 

 showed, the body of a one-celled protozoan 

 may be as complex as that of a many-celled 

 metazoan; and every zoologist knows that 

 a mouse is as complex as an elephant, 

 though it is composed of a much smaller 

 number of cells. In the development of 

 an egg cell the complexity of the entire in- 

 dividual increases only as the number of 

 unlike parts increases; mere duplication 

 of like parts leads to increase in size, but 

 not to increase in complexity. 



Only in relatively simple units is di- 

 vision non-differential so that both prod- 

 ucts are entirely alike, as is probably the 

 case in all ultra-microscopic units, in cell 

 organs and in very simple cells. In more 

 complex individuals, whether they are cells 

 or cell aggregates, the products of division 

 usually differ from one another, at least 

 when first formed, and in the most com- 

 plex individuals division of the entire or- 

 ganism is more or less completely aban- 

 doned. In the division of a protozoan like 

 Paramecium the two products are at first 



unlike, but as they continue to separate 

 they become alike by a process of regula- 

 tion. If these products of fission did not 

 separate and did not undergo regulation, 

 there would be formed a number of cells, 

 organically connected and differing from 

 one another in structure and function. 

 This is just what happens in the cleavage 

 of the egg of a metazoan; the original or- 

 ganism divides into many cells each of 

 which is more or less dependent upon 

 others. The original individual is broken 

 up into many parts, but it is evident that 

 there is one individual of the grade which 

 may be called an organism at the begin- 

 ning of development and just one and no 

 more at its end ; indeed the organism is the 

 same individual from the oosperm to the 

 end of life, irrespective of the number of 

 cells or subordinate parts of which it may 

 be composed. However, if cleavage cells 

 separate and undergo regulation, as in the 

 case of Paramecium, we may have as many 

 organisms as there are separate parts. 

 This applies to the division of groups of 

 cells or body parts as well as to cleavage 

 cells. If cells or parts of cells separate off 

 which are not capable of regulation and of 

 continued life, they do not form independ- 

 ent individuals. 



n 

 If now we inquire what causes an indi- 

 vidual of any grade to divide and thus to 

 give rise to two new individuals we are 

 compelled to confess that we do not know 

 in any instance. The cause of the division 

 of a centrosome or chromosome or nucleus 

 or cell is as mysterious as the cause of di- 

 vision of a hydroid or worm. The division 

 of the cell has been studied more fully 

 than that of any other individual. We 

 know that the centrosome divides before 

 the nucleus and the latter before the cell 

 body, but while we know that a cause must 

 precede its effect we can not say post hoc 



