DEVELOPMENT, INHERITANCE, AND METABOLISM 431 



Claude Bernard regard as but different phases of one process. The 

 building of a definite cell-product, such as a muscle-fibre, a nerve- 

 process, a cilium, a pigment-granule, a zymogen-granule, is in the last 

 analysis the result of a specific form of metabolic activity, as we may 

 conclude from the fact that such products have not only a definite 

 physical and morphological character, but also a definite chemical 

 character. In its physiological aspect, therefore, inheritance is the 

 recurrence, in successive generations, of like forms of metabolism ; 

 and this is effected through the transmission from generation to gen- 

 eration of a specific substance or idioplasm which we have seen 

 reason to identify with chromatin. The validity of this conception 

 is not affected by the form in which we conceive the morphological 

 nature of the idioplasm whether as simply a mixture of chemical 

 substances, as a microcosm of invisible germs or pangens, as assumed 

 by De Vries, Weismann, and Hertwig, as a storehouse of specific fer- 

 ments as Driesch suggests, or as a complex molecular substance grouped 

 in micellae as in Nageli's hypothesis. It is true, as Verworn insists, 

 that the cytoplasm is essential to inheritance ; for without a specifi- 

 cally organized cytoplasm the nucleus is unable to set up specific 

 forms of synthesis. This objection, which has already been con- 

 sidered from different points of view, by both De Vries and Driesch, 

 disappears as soon as we regard the egg-cytoplasm as itself a product 

 of the nuclear activity ; and it is just here that the general role of the 

 nucleus in metabolism is of such vital importance to the theory of 

 inheritance. If the nucleus be the formative centre of the cell, if 

 nutritive substances be elaborated by or under the influence of the 

 nucleus while they are built into the living fabric, then the specific 

 character of the cytoplasm is determined by that of the nucleus, 

 and the contradiction vanishes. In accepting this view we admit 

 that the cytoplasm of the egg is, in a measure, the substratum of 

 inheritance, but it is so only by virtue of its relation to the nucleus, 

 which is, so to speak, the ultimate court of appeal. The nucleus 

 cannot operate without a cytoplasmic field in which its peculiar 

 powers may come into play; but this field is created and moulded 

 by itself. 



J. PREFORM ATION AND EPIGENESIS. THE UNKNOWN FACTOR IN 



DEVELOPMENT 



We have now arrived at the farthest outposts of cell-research, and 

 here we find ourselves confronted with the same unsolved problems 

 before which the investigators of evolution have made a halt. For 

 we must now inquire what is the guiding principle of embryological 

 development that correlates its complex phenomena and directs them 



