Dec. 17, 1885] 



NA TURE 



155 



though most useful as temporary advancements of 

 speculation on the problem, such pre-eminently as 

 Darwin's theory of pangenesis have failed hitherto to 

 dispose of it satisfactorily. It is impossible now to 

 believe that every cell of the organism can give off 

 gemmules which exist at all times in all regions of the 

 body, become collected in the generative cells, and are 

 capable of becoming metamorphosed in regular order 

 back again into the different cells of the organism. 



The problem must be considered anew, and the present 

 essay deals not with the entire subject of heredity, but with 

 the fundamental question, How is it that a single cell of the 

 body unites within itself the entire tendencies of inheritance 

 of the whole organism ? There are only two physiologically 

 conceivable possibilities by which germ-cells endowed 

 with such peculiar powers as we know them to possess 

 can be produced. Either the substance of the parent germ- 

 cell after passing through a cycle of changes required for 

 the construction of a new individual possesses the capa- 

 bility of producing anew identical germ cells, or the germ- 

 cells arise as far as their essential and characteristic 

 substance is concerned, not at all out of the body of the 

 individual, but direct from the parent germ-cell. 



It is this latter view which Prof. Weismann holds to 

 be correct, and maintains in the present essay, and which 

 he terms the theory of the continuity of the Germ-plasma. 

 On this theory heredity depends on the fact that a sub- 

 stance of peculiar chemical and even more special mole- 

 cular composition passes over from one generation to 

 another. This is the " germ-plasma,'" the power of which 

 to develope to a complicated organism depends on the 

 extraordinary complication of its minutest structure. At 

 every Ontogenesis a portion of the specific germ-plasma 

 which the parent egg cell contains is not used up in 

 producing the offspring, but is reserved unchanged to 

 produce the germ-cells of the following generation. 



It is plain that this supposition reduces the question of 

 heredity to one of growth. The germ- cells of all succeeding 

 generations being merely pieces of the same substance as 

 the first, and of the same molecular structure, when 

 nourished under similar conditions, must run through a 

 similar series of stages of development, and yield the 

 same final products. 



After combating objections which may be raised to 

 the theory on the score of the heredity of " acquired 

 modifications,'' it is pointed out that the germ-cells on it 

 appear no longer as a product of the body, at least as far 

 as their essential part, the germ-plasm, is concerned ; 

 they are rather to be regarded as something standing 

 opposed to and separate from the entirety of cells com- 

 posing the body, and the germ-cells of succeeding 

 generations are related to one another as are a series of 

 generations of unicellular organisms derived from one 

 another by a continuous course ofsimple division into 

 two. 



Jager's and M. Nussbaum's views approached very 

 near those of Weismann ; but these authors inferred a 

 continuity of the germ-cells themselves. Such a con- 

 tinuity of cells survives at present in but very few cases. 

 In nearly all instances the generations of germ cells start 

 from the parent, as very minute particles of germ-plasma 

 only, to form, nevertheless, the basis of the germ cells of 

 the next generation. 



The author claims for his theory that even should it 

 require to be abandoned in the future, it nevertheless 

 represents a stage in our knowledge of the problem which 

 must be passed through, which must be clearly stated 

 and carefully worked out, whether the future prove it true 

 or false. With this view of it he develops it in three 

 chapters, the first of which deals with the conception of 

 the germ-plasma. 



The Germ-Plasma. — It now seems established that 

 the only actual carrier of the tendency of heredity is the 

 highly organised nuclear substance ; fecundation consists 



in a union of nuclei ; the surrounding cell substance has 

 no immediate participation in the result. E. van Beneden's 

 splendid researches on Ascaris led far towards this con- 

 clusion in showing that the nucleus of the egg cell does 

 not fuse in any irregular manner with that of the sperm 

 cell, but that the nuclear loops of these two bodies 

 arrange themselves opposite one another in regular order, 

 two and two, and thus construct the new nucleus, the seg- 

 mentation nucleus. Van Beneden, as is well known, 

 viewed the two nuclei concerned as half nuclei male and 

 female respectively, the union of which produced an 

 entire nucleus of hermaphrodite nature. Weismann, on 

 the other hand, speaks of " the union of the nuclear sub- 

 stance of the maternal and paternal individual." Stras- 

 burger has shown that in the fecundation of phanerogams 

 the nucleus alone of the sperm cell (pollen tube), not the 

 cell body also, enters the embryo sac to conjugate with the 

 nucleus of the ovicell. Strasburger, led by Van Bene- 

 den's results, concluded that the occurrence of heredity 

 depends on the transmission of a nuclear substance of 

 specific molecular structure. This specific nucleo-plasma 

 of the germ cell is what Weismann terms germ-plasma. 

 This germ-plasma is, however, by no means identical with 

 Nageli's idioplasma. The idioplasma, according to Nageli, 

 is a network which stretches through the entire body, 

 and in fact constitutes the specific molecular basis deter- 

 mining its manner of the body's existence. The general 

 conception of a molecular basis of the organism governing 

 its existence and opposed to the mere nutrient plasma is 

 a fine and original one, and worthy of much merit, but 

 in its detailed development Nageli's theory cannot now 

 be accepted. Even if the cell bodies are everywhere 

 connected by fine outgrowths in all vegetable and animal 

 pluricellular organisms, as recent research seems to 

 show, the network present is one of nutrient plasma, 

 not of idioplasma, for the determining molecular basis is 

 confined to the nuclei which are not so connected. 

 Moreover, there can be no one single substance such as 

 idioplasma of identical composition permeating the whole 

 body. On the other hand each different kind of cell in 

 each organism must contain its specific kind of idio- 

 plasma, or rather nucleo-plasma regulating its peculiar 

 mode of existence. 



The author quite agrees with Strasburger in consider- 

 ing the " specific peculiarities of organisms as centred 

 in the cell-nuclei," and also in many points in his state- 

 ment that " from the nucleus issue forth molecular stimuli 

 into the surrounding cytoplasma, which, on the one hand, 

 govern the processes of change of material in the cells, 

 and, on the other, give to the growth of the cytoplasma 

 conditioned by the nutrition, a certain character peculiar 

 to the species." A valuable confirmation of this position 

 is afforded by A. Gruber's experiment on Infusoria, that, 

 though artificially separated fragments of Infusoria with- 

 out any portion of the nucleus can live for some time, 

 they never are able to regenerate themselves, whereas 

 fragments containing part of the nucleus always do so. 

 The nature of the nucleo-plasma undergoes a regularly 

 ordered series of changes during ontogeny. The simplest 

 view to take is, that at each division of the nucleus the 

 specific plasma of the nucleus divides itself into two 

 halves, differing in their essential composition, so that 

 each resulting cell-body also, its character being deter- 

 mined by the nucleus, becomes re-fashioned. Thus, in the 

 case of any Metazoon, for example, the two first segmen 

 tation spheres would undergo such change that the one 

 would contain only the tendencies of heredity of the 

 endoderm, and the other only those of the ectoderm, 

 and so on throughout. Against such a supposition, how- 

 ever, stands the fact that is observed in instances of 

 indirect division of nuclei during the process of kario- 

 kinesis, each mother nuclear loop of the nuclear plate 

 splits exactly in two lengthways into two halves. Each 

 daughter nucleus thus receives exactly the same supply of 



