CH. xn] GERM-TRACK 173 



gives rise on the one hand to the body or soma, and on the 

 other hand to the cells of the "germ-track" (Keimbahn); 

 the body grows, becomes adult, and finally decays and dies, 

 but the germ-cells have a sort of potential immortality, 

 giving rise to similar germ-cells, as well as to a body, in the 

 next generation. 



This idea, in its original simple form, was soon modified 

 in consequence of such phenomena as budding and regener- 

 ation, in which it was clear that definitely somatic cells 

 could give rise to complete new individuals or parts, and 

 in place of the distinction between soma and germ-track 

 WEISMANN substituted that of somato-plasm and germ- 

 plasm, and supposed that although the somato-plasm or 

 body-plasm could never be converted into germ-plasm, yet 

 ordinary body-cells might retain sufficient germ-plasm to 

 enable them to regenerate lost parts or even to produce a 

 complete individual. It is not our purpose to discuss any 

 further this speculation of WEISMANN'S, which, whether 

 true or false, has had very great influence on the biological 

 thought of the past thirty years, but rather to consider 

 some of the cases in which the differentiation of body-cells 

 from germ-cells may actually be observed. 



In a considerable number of animals, belonging to differ- 

 ent groups, at an early stage of embryonic development 

 certain cells are recognisable which remain distinct from 

 the surrounding cells and ultimately give rise to the gonads. 

 Cells of this kind have been known for many years in 

 Sagitta, in certain Nematodes and Insects, and in some 

 other forms, but only more recently has it been shown that 

 they are cytologically different from the neighbouring cells 

 which give rise to the body tissues. One of the earlier 

 cases in which this was demonstrated was the embryo of 

 the Nematode Ascaris megalocephala investigated by 



