622 



NATURE 



[Oct. 24, 1889 



germ-cells are potentially immortal, in so far as they are able, 

 under favourable circumstances, to develop into a new indi- 

 vidual, or, in other words, to surround themselves with a new 

 body {soma)." (See also p. 158.) 



This statement is further explained on p. 205 : — 



" Strictly speaking, I have therefore fallen into an inaccuracy 

 in maintaining (in former works) that the germ-cells are them- 

 selves immortal ; they only contain the undying part of the 

 organism — the germ-plasm ; and although this substance is, as 

 far as we know, invariably surrounded by a cell-body, it does 

 not always control the latter, and thus confer upon it the 

 character of a germ -cell." 



Similarly, the substance of the body is termed somato- 

 plasm (p. 104). Moreover, the germ-plasm is stated to 

 be localized in the nucleus of the germ-cell (p. 179). 



The first difficulty which presents itself is to understand 

 how the mortal heteroplastides can have been evolved 

 from the immortal monoplastides or homoplastides. 

 The explanation which Prof. Weismann offers is as 

 follows (p. 27) : — 



"Let us now consider how it happened that multicellular 

 animals and plants, which arose from unicellular forms of life, 

 came to lose this power of living for ever. 



"The answer to this question is closely bound up with the 

 principle of division of labour which appeared among multi- 

 cellular organisms at a very early stage, and which has gradually 

 led to the production of greater and greater complexity in their 

 structure. 



"The first multicellular organism was probably a cluster of 

 similar cells, but these units soon lost their original homo- 

 geneity. As the result of mere relative position, some of the 

 cells were especially fitted to provide for the nutrition of the 

 colony, while others undertook the work of reproduction. 

 Hence the single group would come to be divided into two 

 groups of cells, which may be called somatic and reproductive 

 — the cells of the body as opposed to those which are concerned 

 with reproduction. This differentiation was not at first absolute, 

 and indeed it is not always so to-day. Among the lower Metazoa, 

 such as the Polypes, the capacity for reproduction still exists to 

 such a degree in the somatic cells, that a small number of them 

 are able togive rise to a new organism — in fact, new individuals 

 are normally produced by means of so-called buds. Further- 

 more, it is well known that many of the higher animals have 

 retained considerable powers of regeneration ; the salamander 

 can replace its lost tail or foot, and the snail can reproduce its 

 horns, eyes, &c. 



"As the complexity of the Metazoan body increased, the two 

 groups of cells became more sharply separated from each other. 

 Very soon 'the somatic cells surpassed the reproductive in 

 number, and during this increase they became more and more 

 broken up by the principle of division of labour into sharply 

 separated S3'stems of tissues. As these changes took place, the 

 power of reproducing large parts of the organism was lost, while 

 the power of reproducin"; the whole individual became con- 

 centrated in the reproductive cells alone." 



It is clear that this explanation, plausible as it seems 

 to be, leaves untouched the real question at issue ; the 

 question as to how mortal cells could have been evolved 

 from immortal. Prof. Weismann himself seems to have 

 been conscious of this, for on p. 139 he reverts to the 

 subject as follows : — 



"It maybe objected that cells of which the ancestors pos- 

 sessed the power of living for ever, could not have become 

 potentially mortal (that is, subject to death from internal causes) 

 either suddenly or gradually, for such a change would contradict 

 the supposition which attributes immortality to their ancestors, 

 and to the products of their division. This argument is valid, 

 but it only applies so long as the descendants retain their original 

 constitution. But as soon as the products of the fission of 

 a potentially immortal cell acquire different constitutions by 

 unequal fission, another possibility arises. Now it is con- 

 ceivable that one of the products of fission might preserve the 

 physical constitution necessary for immortality, but not the 

 other, just as it is conceivable that such a cell — adapted for 

 unending life — might bud off a small part, which would live a 

 long time without the full capabilities of life possessed by the 



parent cell ; again, it is possible that such a cell might extrude 

 a certain portion of organic matter (a true excretion) which is 

 already dead at the moment it leaves the body. Thus it is 

 possible that true unequal cell-divisions, in which only one half 

 possesses the condition necessary for increasing, may take place ; 

 and in the same way it is conceivable that the constitution of a 

 cell determines the fixed duration of its life, examples of which 

 are before us in the great number of cells in the higher Metazoa, 

 which are destroyed by their functions. . . . But the reproduc- 

 tive cells cannot be limited in this way, and they alone are free 

 from it. They could not lose their immortality, if indeed the 

 Metazoa are derived from the immortal Protozoa, for from the 

 very nature of that immortality it cannot be lost. From this 

 point of view the body, or soma, appears in a certain sense as a 

 secondary appendage of the real bearer of life — the reproductive 

 cells." 



Prof. Weismann here comes to closer quarters with 

 the real question at issue, but still he does not fully face 

 it. He invokes the principle of "unequal fission" to 

 account for the acquisition of " different constitutions " by 

 the products of fission, but he offers no explanation what- 

 ever of the modus operandi of unequal fission. He makes 

 no suggestion as to the constitution of the body of the 

 Protozoa ; whether it consists, in his opinion, entirely of 

 germ-plasm, and if not, whether or not the germ-plasm is 

 localized in the nucleus. The only criticism which can 

 be made is that the bare mention of "unequal fission" is 

 not a sufficient answer to the objection "that cells, of 

 which the ancestors possessed the power of living for 

 ever, could not have become potentially mortal." It 

 appears to me that any satisfactory answer to this objec- 

 tion must include the assumption that the immortal an- 

 cestors already contained a substance which was poten- 

 tially mortal. It is impossible to conceive that unequal 

 fission can take place in a cell consisting throughout of 

 essentially the same kind of substance. 



Very much the same difficulty presents itself in con- 

 nection with the development of the embryo from the 

 ovum or germ-cell ; in the one case it is phylogenetic, in 

 the other ontogenetic. Prof. Weismann goes into far 

 greater detail in this latter case, and the statements which 

 he makes concerning it may perhaps be intended to 

 throw some light on the former. 



The germ-cell, as pointed out above, is characterized 

 by containing germ-plasm ; and this germ-plasm is 

 localized in the nucleus. There is one point which Prof. 

 Weismann does not mention, and that is as to the nature 

 of that portion of the germ-cell (including the cytoplasm 

 and part of the nucleoplasm) which does not consist of 

 germ-plasm. Of what, then, does it consist? It must 

 consist of somatoplasm : there is no alternative. The 

 germ-cell, then, consists mainly of mortal somatoplasm, 

 and contains in its nucleus a certain amount of immortal 

 germ-plasm. But, as shown in preceding quotations, 

 Prof. Weismann holds that the whole germ-cell is im- 

 mortal. In view of the constitution of the germ-cell, this 

 view seems to be paradoxical, but it appears to be ex- 

 plained on the assumption that the substance of the 

 nucleus determines the nature and character of the cell, 

 though Prof. Weismann does not altogether commit 

 himself to this assumption (see pp. 185, 205, 210). 



From this point of view Prof. Weismann's suggestion 

 that the development of mortal from immortal cells is due 

 to unequal fission, seems to be quite intelligible, not only 

 ontogenetically, but also phylogenetically, if we venture 

 to assume that the constitution of a Protozoon is essen- 

 tially the same as that of a germ-cell. It is easy to 

 imagine that the nucleus of a Protozoon may be divided 

 into two parts, one of which contains the whole or the 

 greater part of the parental germ-plasm, the other con- 

 taining none or only a small portion of it ; the two re- 

 sulting cells would be respectively immortal and niorta', 

 and, supposing they remained coherent, would represent 

 the reproductive and somatic portions of a heteroplastid 

 body. Similarly, if such a division of the nucleus of the 



