THE GERMINAL TRACT 29 



be formed of a number of living particles known as biophors. 

 The biopbors are the ultimate units of organic matter. Tbeir 

 existence is, indeed, hypothetical, but it is a necessary hypo- 

 thesis ; for, as no single molecule of inert matter-"- can of itself 

 determine life, we must suppose a peculiar and complex com- 

 bination of molecules which go to form the li-ving biophor. 

 Biophors, determinants, and ids constitute, therefore, what we 

 may caU the organic hierarchy. 



Before entering into more details concerning the role of the 

 determinants^, it may be useful to glance at the question of the 

 origin of the germ-cells in the developmental process. According 

 to Weismann, the germ-cells do not always arise at the end, but 

 sometimes at the beginning of the embryogenesis, at the same 

 time as the somatic cells. In the Daphnids, the germ-cells are 

 distinguishable in the segmented egg itself, whereas in the case 

 of the human species they are not distinguishable until the 

 process of embryonic development. In cases like the last, 

 the continuity between the parental germ-plasm and that of the 

 offspring can only be established through a lineage of successive 

 somatic ceUs, which contain, along -with active germ-plasm, or 

 somatic idioplasm, a reserve of inactive germ-plasm. The line 

 of continuity which is thus established between the parental 

 germ-plasm and the differentiated germ-cells of the offspring is 

 kno-wn as the " germinal tract " (Keimbahn). Observations on 

 HydromeduscB have shown that only a well-defined lineage of 

 cells is capable of thus transmitting the reserve of inactive germ- 

 plasm. 



While the inactive germ-plasm, the vehicle of heredity, re- 

 mains unaltered structurally during the life of the indi-vidual, 

 the active germ-plasm, located in the body, is used up in the 

 ontogenesis. The function of the active germ-plasm is the 



^ Inert matter is not an altogether accurate expression. The ciurious 

 " brownian movements " and other phenomena make it probable that 

 inorganic bodies are potentially as mobile as organic ones ; the difference 

 is probably merely quantitative. 



