292 



SCIENCE. 



[X. S. Vol. XXI. Xo. 530. 



characters are in some unknown manner 

 represented by corresponding chromosome- 

 characters. We can hardly imagine at 

 present how this is possible ; and it must be 

 freely admitted that such a conclusion has 

 an appearance of artificiality and crudeness 

 that almost inevitably creates a certain feel- 

 ing of scepticism. Nevertheless, to a con- 

 clusion similar in principle to this the facts 

 seem to be pretty definitely pointing. 



And now, finally, let us see how this con- 

 ception, if accepted, is to be united with 

 that of specific protoplasmic stuffs, as al- 

 ready outlined. We do not know in any 

 positive way, but we may roughly present 

 the facts to our minds by a kind of artifi- 

 cial hypothesis— somewhat as Ehrlich and 

 his followers endeavor to present the side- 

 chain theory of immunity by means of 

 rough and crude diagrams. Let us assiame, 

 for example, that the specific protoplasmic 

 stutfs are formed one after another by 

 means of substances like enzymes that em- 

 anate from corresponding chromosomes.* 

 Putting the matter in the sharpest and 

 crudest way, let us assume that each of 

 the chromosomes in our diagram is re- 

 sponsible for the formation of the stuff 

 correspondingly shaded. A few of these 

 stuffs, formed and distributed as the egg 

 ripens, determine the initial stages of de- 

 velopment. In later stages other stuffs are 

 formed by other chromosomes and progres- 

 sively distributed to the cells by division. 

 Thus the cleavage-mosaic grows progres- 

 sively more complex and definite as devel- 

 opment advances. Each nucleus still con- 

 tains the germ or potentiality of the whole 

 organism, but the cells assume specific 

 characters according to the protoplasmic 

 stuffs allotted to them (Fig. 3, E). 



This attempt to portray briefly the modua 

 operandi of development is doubtless an 

 exce.ssive]y nai've mode of formulating a 



* C{. Dricsclrs ' Ferment- Fiktion,' Analyl. 

 Tlieoi ie, pp. 87-92. 



highly complex and subtle process, con- 

 cerning the real nature of which we still 

 know very little. Even if literally correct 

 it would still leave quite out of account 

 some of the most important elements of our 

 problem. I do not offer it as a well-estab- 

 lished or fully rounded conclusion, but 

 rather as a convenient way of placing be- 

 fore you one fundamental result, towards 

 which I believe the drift of recent research 

 is tending. This is that the germ consists 

 of two elements, one of which undergoes a 

 development that is essentially epigenetic, 

 while the other represents an original con- 

 trolling and determining element. The 

 first is represented by the protoplasm of 

 the egg. The second is the nucleus, which, 

 as I have attempted to .show, must appar- 

 ently be conceived as a kind of microcosm 

 or original preformation, consisting of ele- 

 ments which correspond, each for each, to 

 particular parts or characters of the fu- 

 ture organism. The actual development 

 of the embryo, which is manifested by pro- 

 gressive changes in the protoplasm, is by 

 epigenesis, as Harvey and Wolff main- 

 tained. Its primary determination is by 

 means of a preformed apparatus, handed 

 on to the egg from preceding generations 

 in the nucleus, which, though not in any 

 sense a miniature model of the adi;lt. yet 

 somehow embodies in infinitesimal com- 

 pass, the heritage of the race. And thus the 

 most recent discoveries in this difficult field 

 of research are bringing us to a position 

 which can hardly be better stated than in 

 the words written by Huxley more than 

 thirty years ago : ' ' The process which in 

 its superficial aspect is epigenesis appears 

 in essence to be evolution, * * * and de- 

 velopment is merely the expansion of a 

 potential organism or original preforma- 

 tion according to fixed laws." We should 

 not, with the advantage of our present 

 standpoint, read into these words of Hux- 

 ley's a meaning which it was impossible 



