NATURE 



26.S 



HEREDITY. 

 Das Keimplasma. Eine Theorie der Vererbun^. By- 

 August Weismann, Professor in Freiburg i. B. (Jena : 

 Gustav Fischer, 1892.) 



IN the substantial volume whose title stands at the head 

 of this article Prof. Weismann has gathered together 

 the results of the essays and researches which he has given 

 to the world during the last eleven years, and he now 

 presents us with a theory of heredity, for which his pre- 

 vious writings have been but preparatory. 



Those who have followed the German philosopher 

 since the appearance of " Die Dauer des Lebens " in 

 1882 have traversed a wide territory. They have seen 

 many theories and hypotheses come into view and attain 

 a certain degree of definiteness. Some of these are now 

 lost to sight, others have changed their outline and 

 altered the relative proportion of their parts, but, what- 

 ever was the standpoint, the central feature of Prof. 

 Weismann's theory of heredity — the continuity of the 

 germ-plasma— remained unchanged. 



The work before us consists of an introduction, partly 

 historical, and of four books, in which the theory and its 

 application to various biological problems are set forth 

 in the fullest possible manner, and in language for the 

 most part free of technical phrases, so that a non-scien- 

 tific reader can easily follow the argument. At the close of 

 the volume are a summary of the four books and an index. 

 The latter is a novelty in a German book of this kind, for 

 which we cannot be too grateful to its compiler, Fraulein 

 Diestel. 



The title of the work strikes the key-note of 

 Prof. Weismann's theory. Heredity, according to 

 his view, is brought about by the transference 

 from one generation to another of a substance with a 

 definite and very complex constitution, the germ-plasma. 

 This substance is the material basis of heredity ; and it 

 is supposed that a part of it in each reproductive cell is 

 not used up in the construction of the offspring, but is 

 transmitted unchanged to form part of the reproductive 

 cells of the following generation. As the author points 

 out, his theory might be termed " Blastogenesis," or the 

 origin from the germ-plasma, as opposed to Darwin's 

 Pangenesis, or the origin from all parts. 



This germ-plasma is necessarily a most complicated 

 substance ; it cannot arise anew, but can grow and in- 

 crease. Its ultimate constituents, according to the latest 

 view of the Freiburg professor, are certain units termed 

 biophores, which possess the properties of assimilation, 

 growth, and multiplication by means of fission. So long 

 as the organic world consisted of biophores living either 

 singly or united in colonies, heredity and multiplication 

 with subsequent growth were one, since each division 

 resulted in two similar halves, which by growth gave rise 

 to organisms exactly like the parent. 



But, when the principle of the division of labour made 



itself felt, the biophores became differentiated, and simple 



division no longer sufficed to give rise to two similar 



organisms, each exactly like their parent. A special 



NO. I 2 12. VOL. 47] 



mechanism was needed to bring about heredity, and 

 according to our author this was found in the nucleus. 

 This differentiated part of the cell was originally a collec- 

 tion of reserve biophores. After the division of the 

 nucleus the biophores in each half multiplied, and so re- 

 placed those which had separated from them, and thus 

 rendered possible the completion of the new organism. 

 The structure of the nucleus became still more compli- 

 cated when amphimixis, the mingling of the hereditary 

 substance of two individuals, made its appearance. 

 Amongst multicellular organisms, with their endless 

 variety of cells, the same mechanism exists, but in a still 

 more complicated form. Sexual reproduction has arisen ; 

 that is, the "Aniagen " for the whole organism are collected 

 into a single reproductive cell, two such cells come to- 

 gether, and the resulting fertilized egg cell contains the 

 hereditary substance of two individuals. 



According to Prof. Weismann's present view, this here- 

 ditary substance or germ-plasma consists ultimately of 

 biophores. These units are built up in a definite order 

 and arrangement into units of the second degree, which 

 are termed "cell determinants," or simply "determin- 

 ants." Every cell of a multicellular organism is domin- 

 ated in its histological character, and in the rhythm and 

 nature of its division by one of the determinants. Each 

 cell has not, however, a corresponding determinant in the 

 germ-plasma, but cells of the same sort, as, for instance, 

 blood corpuscles, may be represented in the hereditary 

 substance by the same determinant. In the germ-plasma 

 of any species there must be as many determinants as 

 there are variable cells or cell groups in the organism. 

 The determinants are arranged in a definite order in 

 units of the third degree termed " ids." 



These ids, which will be familiar to readers of Prof. 

 Weismann's latest essay, that on Amphimixis, are again 

 built up into idants, and these last probably correspond 

 with the chromatosomes, or nuclear rods of the nucleus. 

 Thus the germ-plasma consists of idants ; the idants are 

 composed of ids ; the ids are built up in a definite man- 

 ner of determinants ; and the determinants are formed 

 of the final units, the biophores. 



It is assumed that the biophores can pass out of the 

 nucleus and divide and multiply in the surrounding cell 

 protoplasm. Amphimixis brings about the mingling of a 

 certain number of biophores from one parent with a 

 similar number from the other, and the organism 

 whose body is dominated by this mixed assemblage of 

 biophores partakes of the nature of both parents. 



In the reproductive cells of the higher plants and 

 animals the nucleus must contain not only those deter- 

 minants which dominate the structure, rate of division, 

 &c., &c., of the cell itself, but also those which will 

 dominate the various cells and systems of cells which 

 will eventually arise from the reproductive cell ; and 

 furthermore these must be definitely arranged in a given 

 order, so that they may not be all called into activity at 

 once, but may become functional successively, in accord 

 with the origin of the cells they control. 



It is not possible within the limits of a short article to 

 describe the many ingenious applications of his theory, 

 by means of which Prof. Weismann attempts to explain 

 such phenomena as the Regeneration of lost parts, 

 Reversion, Budding, Alternation of Generations, &c. : all 



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