The Animal Organism and 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 endodemi of the fully develo})cd 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 nuist contain two quite 

 distinct ])arts : 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 which the journey is accomplished. 



The first part of the theory starts from the indubitable 

 facts that in those hydromedusa' 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 which later develop into medus.'e; 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 endoderm. 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 poly})oid 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 Marscliroute or geiTninal highway or 

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

 question of where this Marscliroute runs, with reference to 

 the germ-layers is what immediately concerns us. 



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

 sex-c^lls passes first into certain cejls of the ectoderm of 



