40 



HARDWICKE'S SCIENCE-GOSSIP. 



existence of Mr. Galton's gemmules, let us see how 

 he explains the processes of heredity. 



Of the whole collection of gemmules in the stirp of 

 any organism, derived from various ancestors in 

 various proportions, comparatively few achieve de- 

 velopment. Of the few which do, each develops 

 into an organic unit of the adult. The conditions 

 which determine the development of the individual 

 gemmules are many and complex, and a great 

 number of struggles between and rearrangements of 

 the different varieties of gemmules representing the 

 same unit take place before a position of equilibrium 

 is attained. Obviously, on the whole, the process 

 will result in a natural selection of the strongest and 

 most suitable gemmules. The residue of gemmules, 

 after this segregation has been effected, remains 

 latent during the life of the individual, and from this 

 residue the sexual elements are derived. 



Professor Weismann's idea of heredity is that it is 

 " brought about by the transference, from one gene- 

 ration to another, of a substance with a definite 

 chemical, and above all molecular, constitution."* 

 This fundamental substance, the germ-plasm, has a 

 very complex structure. At the beginning of the 

 process of segmentation in the development of each 

 individual a certain portion is segregated and remains 

 ■unchanged, to be handed on to the next generation 

 (Galton's residual gemmules) ; the rest undergoes 

 such changes during the process of growth of the 

 developing organism that it directs and determines 

 the construction of the body of the latter. Thus 

 each generation has an identical starting-point, and 

 would be expected under the same conditions to give 

 rise to an identical result. 



Here we recognise the same idea of continuity that 

 we find in Mr. Galton's theory. But we must next 

 inquire what Professor Weismann means by germ- 

 plasm, and we soon discover that his conception of 

 this substance differs essentially from Mr. Galton's. 

 The idea of particulate inheritance did not compel 

 Professor Weismann (as it had done Darwin and 

 Galton) to suppose that separate gemmules, each 

 giving rise to an organic unit of the body, existed in 

 the germ-cells. " The germ-plasm is that part of a 

 germ-cell of which the chemical and physical 

 properties — including the molecular structure — 

 enable the cell to become, under appropriate con- 

 ditions, a new individual of the same species." f As 

 it appears that the essential feature in fertilisation is 

 the fusion of the male and female pronuclei, we must 

 localise this germ-plasm in the nucleus of the germ- 

 cell. Indeed, in the case of flowering-plants the male 

 nucleus only enters the egg-cell. Professor Weismann 

 further takes over Niigeli's conception of idioplasm 

 which we have already explained. He does not, 

 however, follow Nageli in regarding the idioplasm as 



[ Essays on Heredity ' 

 ' Ibid., p. 174. 



(first edition), p. 16 



a solid network extending throughout the organism, 

 but considers that it, like the germ-plasm, is confined 

 to the nucleus. There is a great deal of evidence 

 accumulated during the last ten or fifteen years to 

 show the supreme importance of the cell-nucleus in 

 the nutrition and general economy of the cell. This 

 would hardly be the place to enter into a considera- 

 tion of this evidence, but it certainly seems sufficient 

 to justify the hypothesis that the substance which 

 determines the specific character and functions of the 

 cell resides in the nucleus, and this conception is 

 likewise supported by the fact that the nuclear 

 substance of all the cells of the body is directly 

 derived from the nuclear substance of the fertilised 

 ovum, and as we have already seen, it is almost 

 certainly this nucleus which contains the hereditary 

 tendencies. The term idioplasm then, in Weismann's 

 sense, is applied to the whole of the controlling 

 substance of the organism. This is situated in the 

 nuclei, and gradually changes during the course of 

 ontogeny from the small amount of very complex germ- 

 plasm to the very much larger amount of relatively 

 simple idioplasm of various kinds situated in the cells 

 of the fully differentiated parts of the adult organism. 

 At each cell-division during the course of develop- 

 ment a simplification and differentiation of its 

 structure takes place, till from possessing, as germ- 

 plasm, all the complex potentialities of the entire 

 organism, the idioplasm of the adult comes to consist 

 of as many different varieties as there are different 

 kinds of cells in the body. The idioplasm of each 

 ontogenetic stage is of such a molecular structure 

 that it not only contains the potentialities of all those 

 tissues to which it will ultimately give rise, but that 

 it also must undergo the differentiation and simplifica- 

 tion at the next cell-division necessary to transform it 

 into idioplasm of the next stage. Thus, for instance, 

 the germ-plasm of the first segmentation-nucleus 

 (nucleus of the fertilised ovum) not only contains the 

 potentialities of the whole organism, but is also of 

 such a structure and in such a condition that it must 

 undergo a certain differentiation at the first nuclear 

 division, a differentiation which gives to the first two 

 daughter-nuclei the potentialities of the ectoderm and 

 endoderm, or of the front and hinder part of the 

 body, respectively.* This process goes on in 

 precisely the same manner throughout ontogeny, 

 until finally we arrive at the characteristic cells of the 

 various tissues with their relatively simple but widely 

 differentiated idioplasms. 



The divisions of the nuclei corresponding to those 

 cell-divisions which only result in the production of 

 two daughter-cells similar to the mother-cell, may be 

 distinguished as equivalent . nuclear divisions, as 



* It should be mentioned that it has been found that by 

 destroying one of the first two segmentation spheres of the 

 frog, only the front or hind part of the body, as the case may 

 be, has been able to continue development (which has, of 

 course, soon been arrested), thus proving the separation at the 

 first cell-division of the potentialities of these regions. 



