940 ^LZVr AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Gallon/ Brooks, 2 and de Vries. 3 But neither the original theory nor its 

 modifications have been generally accepted. 



Weismann' s Theory. Since 1880, Professor Weismann 4 of Freiburg has 

 published numerous essays upon heredity and allied subjects, in which, besides 

 reviewing the views of others, he has developed in detail a new and elaborate 

 theory of his own, that is the most ambitious attempt yet made to solve the 

 problem of inheritance. In the course of their development Weismanu's 

 ideas have undergone some modification. Their leading features are as 

 follows : 



The essential hereditary substance, or germ-plasm, is the chromatin of the 

 nucleus of the germ-cells. One of the fundamental tenets of Weismann's 

 system is expressed by his own phrase, " the continuity of germ-plasm." By 

 this is meant that the germ-plasm of one individual, instead of arising de novo 

 in the individual by the collocation of multitudinous " gemmules " derived 

 from the body-cells, originates directly from the germ-plasm of the parent, 

 thence from that of the grandparent, and so on backward through all genera- 

 tions to the origin of all germ-plasms that took place simultaneously with the 

 origin of sex germ-plasm is continuous from individual to individual along 

 any one line of descent. Weismann draws a sharp line between germ-plasm 

 and somatoplasm, or body-plasm, which latter comprises all protoplasm that 

 the body contains except the germ-plasm. Germ-plasm once originated con- 

 tinues from generation to generation ; somatoplasm develops anew in each gen- 

 eration from germ-plasm by growth and differentiation, resulting in a loss of its 

 specific germinal character. Germ -plasm is stable in composition ; somatoplasm 

 is variable. Germ-plasm, being passed on from parent to offspring, is immortal ; 

 somatoplasm dies when the individual dies. Weismann believes that "the 

 germ-plasm possesses a fixed architecture, which has been transmitted histori- 

 cally " and which represents the parts of the future organism. It consists of 

 material particles or hereditary units called determinants, each of which has a 

 definite localized position within the germ-plasm. The determinants are sug- 

 gestive of Darwin's gemmules, yet they are not the same, for, while gemmules 

 were supposed to represent individual cells, determinants are representatives 

 of cells or groups of cells that are variable from the germ onward. Deter- 

 minants consist of definite combinations of simpler units, or biophors, which 

 are the smallest particles that can exhibit vital phenomena. Below biophors 

 there come in order of simplicity of material structure the molecules and 

 the atoms of the physicist. Above biophors and determinants Weismann 

 finds it necessary to assume the existence of higher units, named in order ids 

 and idants, the former being groups of determinants, and actually visible as 

 granules of chromatin, the latter being the chromosomes of the nucleus. Each 



1 Francis Galton : "A Theory of Heredity," Journal of the Anthropological Institute, 1875. 



2 W. K. Brooks: The Laws of Heredity, 1883. 



3 H. de Vries : Die IntraceUulare Pangenesis, 1889. 



4 August Weismann : Essays upon Heredity and Kindred Biological Problems, authorized 

 translation, vol. i., 1889; vol. ii., 1892; The Germ-plasm, authorized translation, 1893; The 

 Effect of External Influences upon Development, the Koraanes Lecture, 1894. 



