THE FORMATION OF GERM-CELLS 185 



Daphnido'. In these animals the primary germ-cells become 

 separated from the somatic cells in the first stages of the seg- 

 mentation of the egg ; and in Sagitta again, this separation 

 takes place at the gastnila-stage. In Vertebrates this process 

 occurs much later, although it always takes place within the 

 first half of embryogeny ; while in Hydroids — both in colonial 

 and solitary forms — the germ-cells do not appear in the ' per- 

 son ' which is developed from the ovum at all, and only arise in a 

 much later generation, which is produced from the first by con- 

 tinued budding. The same is true as regards the higher plants, in 

 which the first shoot arising from the seed never contains orerm- 

 cells, or even cells which subsequently become differentiated 

 into germ-cells. In all these last-mentioned cases the germ- 

 cells are not present in the first person arising by embryogeny 

 as special cells, but are only formed in much later cell-genera- 

 tions from the offspring of certain cells of which this first person 

 was composed. These ancestors of the germ-cells cannot be 

 recognised as such : they are somatic cells, — that is to say, they, 

 like the numerous other somatic cells, take part in the construc- 

 tion of the body, and may be histologically differentiated in 

 various degrees. 



A series of organic species might therefore be formed in 

 which the formation of the germ-cells begins at very different 

 degrees of remoteness from the egg-cell. This would admit of 

 the interpretation that the fertilised egg-cell of the earliest 

 Metazoa first divided into two cells, one serving for the forma- 

 tion of the body (soma), and the other for that of the germ- 

 cells ; and that a shifting had occurred subsequently, owing 

 to a separation of the material for both parts in the germ-plasm, 

 so that the portion of the germ-plasm Avhich remained un- 

 changed was supplied in an inactive condition, in the form of 

 accessory idioplasm, to one of the somatic halves of the egg- 

 cell, and was transmitted by the latter to a somatic cell of 

 the second, third, or fourth generation. The shifting of the 

 process of separation into germ-cells and somatic cells finally 

 reached its extreme limit, as in the case of the Hydroids, and 

 the unchanged germ-plasm of the fertilised ovum then only led 

 to the formation of germ-cells after passing through a long series 

 of somatic cells. 



These facts do not, however, as yet constitute an actual proof 

 of the correctness of this interpretation : they might be taken 



