Oct. 24, 1889] 



NATURE 



623 



germ-cell took place, the two resulting cells would repre- 

 sent the reproductive and somatic portions of the body 

 of the embryo. 



But this does not appear to be Prof. Weismann's view 

 of embryogeny. On the contrary, he holds strongly that 

 the germ-plasm of the ovum gives rise, in part at least, to 

 the somatoplasm of the embryo. Thus, on p. 168 he 

 says : — 



"I have called this substance 'germ-plasm,' and have assumed 

 that it possesses a highly complex structure, conferring upon it 

 the power of developing into a complex organism. I have 

 attempted to explain heredity by supposing that in each onto- 

 geny a part of the specific germ-plasm contained in the parent 

 egg-cell is not used up in the construction of the body of the 

 offspring, but is preserved unchanged for the formation of the 

 germ- cells of the following generation." 



It is not a little remarkable that Prof. Weismann 

 should not have offered any suggestion as to the concep- 

 tion which he has formed of the mode in which the 

 conversion of germ-plasm into somatoplasm can take 

 place, considering that this assumption is the key to 

 his whole position. He has been at considerable pains 

 to controvert the view that somatoplasm may be con- 

 verted into germ-plasm ; but in making this attack he 

 has overlooked the necessity for defence. There is really 

 no other criticism to be made on an unsupported assump- 

 tion such as this, than to say that it involves a contradic- 

 tion in terms. The idea of the conversion of germ-plasm 

 into somatoplasm is quite as impossible as that of the 

 conversion of somatoplasm into germ-plasm. It is 

 absurd to say that an immortal substance can be con- 

 verted into a mortal substance. If such an apparent 

 change takes place, the only possible conclusion is that 

 the so-called immortal substance was never truly im- 

 mortal, inasmuch as it must have always possessed the 

 potentiality of mortality. 



It may perhaps be represented that the foregoing criti- 

 cisms are altogether of too minute and detailed a cha- 

 racter to affect the general validity of Prof. Weismann's 

 argument. My answer is that I understand Prof. Weis- 

 mann to imply that his theory of heredity is not — like, for 

 instance, Darwin's theory of pangenesis — " a provisional 

 or purely formal solution" (Weismann, p. 166) of the 

 question, but one which is applicable to every detail of 

 embryogeny, as well as to the more general phenomena 

 of heredity and variation. 



We may now proceed to the consideration of Prof. 

 Weismann's theory of heredity. The essential features 

 of it are given in the following paragraphs (p. 73) : — 



" Among these unicellular organisms, heredity depends upon 

 the continuity of the individual during the continual increase of 

 its body by means of assimilation. 



" But how is it with the multicellular organisms which do not 

 reproduce by means of simple division, and in which the whole 

 body of the parent does not pass over into the offspring ? 



" In such animals the power of reproduction is connected with 

 certain cells, which, as germ-cells, may be contrasted with those 

 which form the rest of the body ; for the former have a totally 

 different role to play ; they are without significance for the life 

 of the individual (that is, for the preservation of its life), and 

 yet they alone possess the power of preserving the species. Each 

 of these can, under certain conditions, develop into a complete 

 organism of the same species as the parent, with every indi- 

 vidual peculiarity of the latter reproduced more or less com- 

 pletely. How can such hereditary transmission of the characters 

 of the parent take place? How can a single reproductive cell 

 reproduce the whole body in all its details ? " 



Prof. Weismann's answer to these questions is as 

 follows : — 



" We have an obvious means by which the inheritance of all 

 transmitted peculiarities takes place, in the continuity of the 

 substance of the germ-cells, or germ-plaint. If, as I believe, 

 the substance of the germ-cells, the germ-plasm, has remained 



in perpetual continuity from the first origin of life, and if the 

 germ-plasm and the substance of the body, the somatoplasm, 

 have always occupied different spheres, and if changes in the 

 latter only arise when they have been preceded by corresponding 

 changes in the former, then we can, up to a certain point, un- 

 derstand the principle of heredity ; or, at any rate, we can con- 

 ceive that the human mind may at some time be capable of 

 understanding it " (p. 104). 



" Now if it is impossible for the germ-cell to be, as it were, 

 an extract of the whole body, and for all the cells of the organ- 

 ism to despatch small particles to the germ-cells, from which 

 the latter derive the power of heredity ; then there remain, as 

 it seems to me, only two other possible, physiologically conceiv- 

 able, theories as to the origin of germ-cells, manifesting such 

 powers as we know they possess. Either the substance of 

 the parent germ-cell is capable of undergoing a series of changes 

 which, after the building-up of a new individual, leads back 

 again to identical germ-cells ; or the germ-cells are not derived 

 at all, as far as their essential and characteristic substance i 

 concerned, from the body of the individual, but they are derived 

 directly from the parent germ-cell. 



"I believe that the latter view is the true one 



propose to call it the theory of the ' continuity of the germ 

 plasm,' for it is founded upon the idea that heredity is brough 

 about by the transference, from one generation to another, of a 

 substance with a definite chemical, and, above all, molecula 

 constitution. I have called this substance 'germ-plasm,' and 

 have assumed that it possesses a highly complex structure, con- 

 ferring upon it the power of developing into a complex organ- 

 ism. I have attempted to explain heredity by supposing that in 

 each ontogeny a part of the specific germ-plasm contained in 

 the parent egg is not used up in the construction of the body of 

 the offspring, but is reserved unchanged for the formation of the 

 germ-cells of the following generation " (p. 167). 



" I believe that heredity depends upon the fact that a small 

 portion of the effective substance of the germ, the germ-plasm, 

 remains unchanged during the development of the ovum into an 

 organism, and that this part of the germ-plasm serves as a 

 foundation from which the germ-cells of the new organism are 

 produced. There is, therefore, continuity of the germ-plasm 

 from one generation to another. One might represent the germ- 

 plasm by the metaphor of a long creeping root-stock from which 

 plants arise at intervals, these latter representing the individuals 

 of successive generations " (p. 266). 



This theory appears to fully account for the transmis- 

 sion and maintenance of ancestral characters ; but of 

 course it depends on the assumption that the germ-plasm 

 is a substance of great stability. This is, in fact. Prof. 

 Weismann's view (p. 271) : — 



' ' The germ-p!asm, or idioplasm of the germ-cell (if this latter 

 term be preferred), certainly possesses an exceedingly complex 

 minute structure, but it is nevertheless a substance of extreme 

 stability, for it absorbs nourishment, and grows enormously 

 without the least change in its complex molecular structure." 



In spite of the simple, and apparently satisfactory, 

 explanation of the phenomena of heredity which this 

 theory affords, there are, nevertheless, serious difficulties 

 in the way of its acceptance. It is open to criticism 

 eyen from Prof. Weismann's own standpoint. The fate 

 of the germ-plasm of the fertilized ovum is, according to 

 Prof. Weismann, to be converted in part into the somato- 

 plasm of the embryo, and in part to be stored up in the 

 germ-cells of the embryo. This being so, how are we 

 to conceive that the germ-plasm of the ovum can impress 

 upon the somatoplasm of the developing embryo the 

 hereditary character of which it (the germ-plasm) is the 

 bearer ? This function cannot be discharged by that 

 portion of the germ-plasm of the ovum which has become 

 converted into the somatoplasm of the embryo, for the 

 simple reason that it has ceased to be germ-plasm, and 

 must therefore have lost the properties characteristic of 

 that substance. Neither can it be discharged by that 

 portion of the germ-plasm of the ovum which is aggre- 

 gated in the germ-cells of the embryo, for under these 

 cumstances it is withdrawn from all direct relation with 

 the developing somatic cells. The question remains 

 without an answer. 



