January 17, 1908] 



SCIENCE 



93 



stances appear at an earlier or later stage 

 in the development of all animals, and 

 these substances are then sorted out and 

 localized; this is differentiation. Physi- 

 ological division of labor involves morpho- 

 logical division of substance; sorting out 

 of functions implies sorting out of the ma- 

 terial substratum of functions. 



Unfortunately, we do not know many of 

 the steps by which different substances ap- 

 pear within protoplasm. Even the forma- 

 tion of non-living products, such as oil, 

 yolk and secretions, is but imperfectly 

 understood, while the manner of formation 

 of different kinds of protoplasm is almost 

 wholly unknown. No one doubts, however, 

 that different kinds of protoplasm are 

 formed in the course of development, that 

 the substance of a muscle cell, for ex- 

 ample, is different from that of a nerve 

 cell, and that both are different from the 

 germinal protoplasm; furthermore, no one 

 doubts that the relatively few substances 

 of the germ cells give rise through many 

 transformations to the relatively numerous 

 substances of the adult. 



But although little is known regard- 

 ing the method of origin of the dif- 

 ferent substances which appear in the 

 process of differentiation, in all eases 

 which have been carefully studied one 

 significant fact appears, viz., the impor- 

 tance of the interaction of the nucleus 

 and cytoplasm. It is well known that 

 many differentiations first appear in the 

 immediate vicinity of the nucleus; indeed, 

 in many cases various substances have been 

 seen to come out of the nucleus and to 

 mingle with the cytoplasm, while the 

 nucleus in turn absorbs substances from 

 the cytoplasm. It is known that con- 

 structive metabolism, differentiation and 

 regeneration never occur in the absence of 

 a nucleus. On the other hand, Verworn has 

 shown that the nucleus alone is incapable 

 of performing these functions, and he 



maintains that the chief role in the life of 

 the cell can not be assigned to either the 

 nucleus or the cytoplasm, but that both are 

 concerned in vital phenomena. Judged 

 merely by the results of observation and 

 experiment, and wholly apart from current 

 theories, it must be admitted that there is 

 good reason to believe that the different 

 substances which appear in the differentia- 

 tion of a tissue cell arise through the inter- 

 action of the nucleus and cytoplasm, and 

 not from either of these alone. 



Turning now to the differentiations of 

 the fertilized egg cell, we find that essen- 

 tially the same conditions obtain as in the 

 differentiation of a tissue cell. Here, also, 

 different substances appear in the egg ceU 

 and become localized in different regions 

 of the egg or embryo. In most animals the 

 different kinds of substance in the unseg- 

 mented egg are not numerous nor conspicu- 

 ous, though in all cases so far studied at 

 least three kinds of substance may be sepa- 

 rated by means of the centrifuge. Here, 

 as in the case of the tissue cells, it is known 

 that there is an active interchange of nu- 

 clear and cytoplasmic substances. In the 

 long growth period of the egg the nucleus 

 grows enormously, evidently at the expense 

 of substances received from the cell body. 

 On the other hand, it is well established 

 that substances issue from the nucleus into 

 the cell body and mingle with the cyto- 

 plasm during this stage, and it is generally 

 believed that one of the substances thus 

 formed, the yolk nucleus, is instrumental 

 in the formation of yolk. At the end of the 

 growth period of the egg the nuclear mem- 

 brane dissolves and a relatively enormous 

 quantity of nuclear material is thus liber- 

 ated into the cell body, while an insignifi- 

 cant quantity persists in the form of chro- 

 mosomes and gives rise to future nuclei. 

 During every cleavage of the egg the nu- 

 cleus grows by absorbing substances from 

 the ceU body, only to give back other sub- 



