January 17, 1895] 



NATURE 



267 



human nature, but we cannot in gross, mechanical fashion 

 seek for the organisation of a society in the primitive 

 nature of man. In a like manner the character of the 

 perfected organism is founded on the nature of the cells 

 which compose it, but it contains in itself a new element, 

 a heterogeneity due to correlation and reciprocity, which 

 is limited by the specific nature of the cell substance, 

 but is not to be sought for as a specific constituent of the 

 substance of any individual cell. Starting from this 

 point of view, we may consider Dr. Hertwig's own doc- 

 trines which he sets forth in the second part of his work 

 entitled " Gedanken zu einer Entwicklungstheorie der 

 Organismen." Whereas Weismann seeks the cause of 

 the orderly development of the primordium in the pri- 

 mordial substance itself, Ilertwig considers that the 

 development of the primordium is dependent on con- 

 ditions or causes which lie outside of the primordial sub- 

 stance of the ovicell, but are none the less produced in 

 regular succession during the process of ontogeny. Such 

 are, in the first instance, the reciprocal relations in which 

 cells stand one to another in increasing degrees of com- 

 plexity, whilst they increase in number by division ; and 

 in the second place, the action of the external environ- 

 ment on the organism. The argument in support of this 

 proposition is given in so condensed a form that it is 

 almost impossible to give any part or abstract of it with- 

 out giving the whole. The following sentence is so im- 

 portant that it may be quoted at length, and the reader 

 should refer to the work itself for the rest of the' argu- 

 ment and the conclusion : - 



" One of the most important and essential causes 

 of the appearance of heterogeneity in the course of 

 development is to be found in the specific power 

 of the ovicell, to multiply itself by division. From 

 this fact alone, that the nuclear substance is able, in the 

 course of most manifold chemical processes, to assimilate, 

 step by step, matter from the reserve material stored up 

 in the egg, and oxygen from the surrounding atmosphere, 

 it is able at the same time to evoke an ever-increasing 

 heterogeneity. The increase of mass of the nuclear sub- 

 stance involves its progressive division into 2, 4, 8, 16 

 parts, and so forth. But the division is again the cause 

 of a constantly changing spatial distribution of the sub- 

 stance. The 2, 4, 8, 16 and following nuclei which arise 

 by division give way to one another in opposite direc- 

 tions, and attain to new positions at definite distances 

 from one another within the limits of the egg. Whilst 

 all the material particles of the ovum were at first 

 arranged round the fertilised nucleus as a single centre 

 of force, they are now grouped around as many individual 

 centres as there are new nuclei, and they segregate them- 

 selves around these as cells. It is therefore clear that 

 the ovum as an unicellular organism, when compared 

 with the ovum as a multicellular organism, has aliered 

 Its quality to an important extent, and that merely by 

 the process of isocleronomic division." 



This is what Hertwig calls the function of growth as a 

 form-producing principle. The other principles which he 

 invokes are the relations of the cells to external conditions, 

 or the function of their position, and finally the reciprocal 

 influence of the parts of the whole on one another and on 

 the whole, or the function of correlation. It is not to 

 be denied that these are principles of considerable 

 importance, and it may be said that their importance has 

 never quite been lost sight of, but they do not bring us 

 any nearer to thi explanation of the totally different 

 NO. I 316, VOL. 5 l] 



reaction of apparently similar substances to the same 

 stimuli. What we want to know is why, when we place 

 under a hen her own eggs and those of a duck, and so 

 expose them to identical conditions, chickens and duck- 

 lings are hatched as unlike one another as may be. The 

 answer given, "the difference can be due to nothing else 

 than to the different nature the different micellar struc- 

 ture) of the substance,' is unsatisfactory, in that it is 

 only a restatement of the fact, and is not an explana- 

 tion at all. In attempting to give a more definite 

 account of the different natures of the egg substance, 

 Hertwig is obliged to take his stand upon much the 

 same ground as the evolutionists. " In the hen's egg the 

 species is present as fully as in ths hen, and the hen's 

 egg is as different from the frog's egg as the hen is from 

 from the frog." He is compelled to agree with the 

 evolutionists in assuming the existence of a specific, and 

 even of a very highly organised primordial substance as 

 a basis of developmental processes, but he claims that 

 his concept of this substance is different from and better 

 than theirs, in that he ascribes to the primordial substance 

 or germ-plasm characters which are congruous with the 

 concept and character of a cell, and does not ascribe to 

 it the innumerable characteristics which are only evoked 

 through the union of many cells and the concomitant 

 action of external conditions. The distinction appears to 

 be a slight one, yet on careful consideration it assumes 

 more important dimensions. Hertwig, if I interpret him 

 rightly, conceives of the germ-plasm as a substance of 

 many and definite potentialities. He does not attribute 

 this potentiality to one part, and that to another part of 

 the germ-plasm, but argues that as a result of multiplica- 

 tion by division the relations of the cells with their 

 contained germ-plasm are continually undergoing change, 

 both with regard to one another and to the environment. 

 In consequence of the dilTerent conditions thus induced, 

 this potentiality is evoked in the germ- plasm of one cell, 

 that in the germ-plasm of another, and so on in ever- 

 increasing grades of complexity. The differentiation of 

 any cell is the result of its position, which determines 

 which of its many potentialities shall be called into 

 action. It is the reaction of the living substance to the 

 stimulus which evokes a particular potentiality, which 

 brings about its form changes, and it may be supposed 

 that a profound form change following on constant action 

 in one direction incapacitates the cell in question for the 

 performance of any of the many other duties for which it 

 was primitively fitted. This conception is epigenetic in 

 that it admits the coming into being of a new heterogeneity 

 which was not pre-existent in the ovum as such ; th.^t 

 which was present was a capacity for certain kinds of 

 heterogeneity. \'ery different is the conception of the 

 evolutionists, who define the exact potentialities of the 

 germ-plasm, and assign each to a given material particle 

 or group of particles. The heterogeneity is already 

 present ; thenceforward there is no room for the increa:e 

 of complexity in response to external stimuli. Dr. Hert- 

 wig rightly saystliat Weismann's explanation amounts to 

 nothing more than a renunciation of an explanation. His 

 doctrine of determinants leads us into an invisible 

 world in which there is no foothold for research. 

 For this reason, if for no other, we should welcome 

 Dr. Oscar Henwig's invitation to return to the paths 



