2IO THE GERM-PLASM 



analogous to that of the production of a single polype by budding 

 from a polype-stock. But both these processes are essentially 

 the same as that of the development from the ovum in a higher 

 animal. In all three cases the formation of the new animal 

 originates in one cell. The latter must therefore possess an 

 idioplasm which contains all the primary constituents of the or- 

 ganism : and. moreover, if the organism is to be ' fertile/ — in the 

 sense in which this term is used by botanists. — the original cell 

 from which it is derived must contain the primary constituents 

 of all the structures characteristic of the species in its idioplasm : 

 that is to say, it must contain germ-plasm. If we trace the 

 development of such a shoot or organism, we shall find that it 

 follows a precisely similar course to that which we have already 

 described in the case of embryogeny ; and that at each cell- 

 division the primary constituents break up into smaller and 

 smaller groups, until at last each cell only contains one such 

 element. And yet all these very different kinds of cells are 

 descended in a direct line from the original cells. How, then, 

 can we account for the fact that one or several of them contain 

 all the primary constituents of the species in a latent condition, 

 in addition to one specific primary constituent of a particular 

 kind of somatic cell, as must be the case in those which give rise 

 to germ-cells? It would, indeed, be a very simple matter if a 

 continuous series of cells which contain ' germ-substance ' only, 

 led from the original cell to the new germ-cells. But as simple 

 a case as this only occurs in the Diptera : in all other instances 

 the i}iterniediate cells which constitute the germ-track can be 

 proved to contain perfectly definite somatic elements in addition 

 to the germ-plasm ; and this is the case in plants as well as i)i 

 ajiimals. 



To make this clear, it is only necessary to glance at the genea- 

 logical tree representing the ontogeny of Rhabditis nigrovenosa 

 (fig. 1 6). How does it come to pass, for example, after the 

 division of the primary endoderm cell into the first endoderm 

 and first mesoderm cell, that the latter is nevertheless capable 

 of producing cells subsequently which contain ' germ-substance,' 

 i.e., germ-cells? At its origin this cell gave up the primary con- 

 stituents of the endoderm to the sister-cell ; by what means do 

 these primary constituents — even those of the ectoderm which 

 were previously given up — reach the germ-cells which eventually 

 arise from this cell? My answer to these questions has already 



