259 



feature, but also in mind and temperament, and in that damnosa 

 luereditas liability to particular diseases. 



So far we are dealing with facts with which every physiologist 

 is familiar. 



Now, the size of the segmentation-nucleus, which we have seen 

 is formed by the fusion of particles from both parents, is itself 

 exceedingly minute, and further, in the course of division of the 

 egg-cell the substance of this segmentation-nucleus must in the 

 course of its ditfusion amongst the continuously-increasing cells 

 of the growing embryonic body, undergo a degree of attenuation 

 far surpassing the extremest idea of homceopathic dilution ; yet 

 the presence of these attenuated particles is sufficient to stamp 

 the oftspring with the characters, not only of the immediate 

 parents, but also with those of still remoter ancestors. 



It is to be noted also that the power of production of a new 

 individual lies in the special cells called germ- and sperm -cells 

 respectively, which are, so to speak, isolated from the rest of the 

 body and set apart for this special purpose of reproduction. Even 

 at a very early period of growth they are thus marked off from 

 the rest of the embryonic body, taking no part in its growth, but 

 being only dependent on the constituent cells of this for shelter 

 and nutrition. These special cells constitute the essential por- 

 tions of what are called the reproductive organs. 



The central problem of heredity is therefore the discovery of 

 the manner in which these reproductive cells, marked off' as they 

 are from the rest of the body, become stamped, as it were, with 

 the potentiality of transmitting to the offspring the multifarious 

 characteristics and peculiarities, not only of the parent organisms 

 but also to a lesser degree those of preceding ancestors ; or as 

 Weismann himself puts the question — " How is it that a single 

 cell can reproduce the tout ensemble of the parent with the faith- 

 fulness of a portrait ? " He proceeds to offer for the solution of 

 this extremely difficult problem his theory of the "Continuity of 

 the Germ-plasm," which, according to his views, rests upon the 

 fact that the reproductive cells are not derived from the general 

 body-cells of the individual in which they occur, but are the direct 

 descendants of the germ-cells of the parent. In other words 

 Weismann represents that the particulate body formed from the 

 fusion of elements from both parents contains a substance which 

 he calls germ-plasDi, and this, he states, is not entirely used up in 

 the formation of a given offspring, but a portion is set aside, it 

 may be at an exceedingly early stage of development, and re- 

 served unchanged to form the germ-cells of the next generation. 

 The sharpest distinction is thus to be drawn between two classes 

 of cells composing the body of the developing germ which are des- 

 tined for entirely different purposes. One set is destined for the 



