The Animal Organism a7id its Germ-Layers 61 



The very earliest recognizable sex-cells occur in the one or 

 the other of the two body layers ; that is, in the ectoderm 

 or the endodenn of the fully developed animal. Since Weis- 

 mann's theory denies the possibility of the origin of the 

 sex-cells, or at least the essential part of these, the germ- 

 plasm, from somatoplasm, his theory of sex-cell production 

 in such animals as the hydroids must contain two quite 

 distinct parts : one as to the route by which the germ-plasm 

 travels from parental to filial sex-cell, and the other as to 

 the force or forces by whicli the journey is accomplished. 



The first part of the theory starts from the indubitable 

 facts that in those hydromedus.x' having a free-swimming 

 medusa, or jelly fish, the sex-cells are borne by this and not 

 by the polyp; that these cells do not as a rule arise in the 

 medusa itself, but somewhere in the colony of polyps, from 

 which location they migrate (in some cases for considerable 

 distances) to the buds whicli later develop into medusae; and 

 that in the majority of species which have been examined 

 with reference to the point, the mature sex-cells are found 

 in the ectoderm and not in the endodenn. On the basis of 

 these facts Weismann thought out a very elaborate and in- 

 genious theory by means of which through various assump- 

 tions about the evolutionary history of the hydromedusa?, 

 he was able to make the facts seem not only to harmonize 

 with, but to support positively the doctrine of the con- 

 tinuity of the germ-plasm. Into that part of the theory 

 which concerns the phylogenetic relation of the medusoid to 

 the polypoid forms of the group, we need not. go further 

 than to say that through his speculations on this point he 

 was able to provide a Marschronte or germinal highway or 

 germ track from the parent sex-cell to the filial sex-cell. The 

 question of where this Marschronte runs, with reference to 

 the germ-layers is what immediately concerns us. 



According to the theory, the germ-plasm of the parental 

 sex-cells passes first into certain cells of the ectoderm of 



