60 The Unity of the Organism 



Since it is the main office of the sex-cells to carry the sup- 

 posed germ-plasm, and since germ-plasm cannot be distin- 

 guished from somatoplasm by direct observation, and since 

 sex-cells are not present in the individual organism of most 

 species up to and including the layered stage of its life, the 

 question of exactly how and where the sex-cells arise will 

 be seen to involve the question of which of the layers they 

 arise in. The problem may be stated more explicitly as 

 follows : Where is the germ-plasm between the time when the 

 germ-cell which is the beginning of a new individual disap- 

 pears through division, and the appearance of new germ- 

 cells within that individual, as the first stage in the life of 

 an individual of the next generation? Or, stating the ques- 

 tion still more explicitly, by what means and by what route 

 does the assumed pre-existent germ-plasm get from its orig- 

 inal place in the sex-cell which produces a given individual 

 to the sex-cell produced by that individual? 



Weismanns Studies on the Origin of Germ-Cells m Hydroids 



Our purpose restricts our examination of the question 

 stated in this general form, to the particular question of 

 where and how (relative to the germ-layer) the sex-cells 

 arise in an individual. Since Weismann elaborated his solu- 

 tion of the problem largely on the basis of phenomena pre- 

 sented by the Hydromedusae, many of which phenomena 

 were brought to light by his own researches, we shall give 

 these animals the central place in our examination. In the 

 first place, the fact should be clearly understood that Weis- 

 mann has never contended that a direct observable continu- 

 ity between the parent sex-cell and the sex-cells of the off- 

 spring occurs in this group of animals. On the contrary, 

 he fully recognizes, as do all students of these animals who 

 have occupied themselves with this particular point, that 

 a wide gap separates the parental from the filial sex-cells. 



