THE FORMATION OF OERM-CELLS 1 89 



active migrations of the germ-cells take part in it. Originating 

 in the ectoderm, these cells wander into the endoderm, and 

 subsequently back again into the ectoderm ; and this remark- 

 able process occurs in a definitely prescribed and regular 

 manner. In spite of the relegation of their place of origin to 

 earlier persons of the stock, the germ-cells always originate from 

 tJie sa)ne layer of cells as that from which they arose in the 

 ancestors of the species. It may thus be said that they are 

 developed ontogenetically from the ancestors of those cells from 

 which they would have arisen if the polype stock still produced 

 free medusae ; or, in other words, they arise lower down on 

 the genn-track at present than they did formerly. Thus in 

 Hydractinia echinata, for instance, the youngest egg-cells first 

 become visible in the endoderm of certain polypes in the same 

 regions from which gonophores (degenerate medusae) subse- 

 quently bud out. The egg-cells then migrate into the latter, and 

 enter the ectoderm of the manubrium as soon as it is formed ; 

 and in this way they return to the old place of ripenings which in 

 earlier ti?nes was also the place in which they were formed. At 

 the present day, however, the egg-cells only apparently originate 

 in the endoderm of the polype : it can indeed be proved that 

 they are derived from the ectoderm, but migrate into the endo- 

 derm while still in a very young condition, before they exhibit 

 the definite character of egg-cells. They therefore originate in 

 the same region in which at an earUer phyletic period the 

 ectodermal layer of the manubrium of the medusa was devel- 

 oped ; or, in other words, the same ontogenetic series of cells 

 which produce the egg-cell at the present day did so in former 

 times. This fact probably only admits of one interpretation, 

 and this is, that only certain series of cells contain the primary 

 constitueftts of the germ-cells, and wherever it became useful in the 

 course of phylogeny for the germ-cells to be situated in another 

 position and in another layer of the body-wall, this change of 

 position could only be effected by the ceils of the germ-track 

 becoming transformed into germ-cells at an earlier stage, and at 

 the same time migrating into the other layer of the body-wall. 

 If any — I will not say all — of the cells could give rise to germ- 

 cells, this complicated mode of procedure would be quite inex- 

 plicable, for Nature always takes the shortest possible course. 



If this reasoning is correct, the hypothesis of the germ-tracks, 

 as I have formerlv stated it, is inevitable ; and the fact that the 



