The germ-cells. 683 



outlines the phenomena of the migrations of the like cells of the hydroid 

 polypes from the stock into the sexual "persons". This fact forced 

 itself into prominence in the course of the investigation, and, until it 

 did so, no suggestion of any connection with an antithetic alternation 

 of generations arose. 



Times without number in the course of the microscopical studies 

 the same idea intruded itself, "how like these migrations, this origin 

 outside the sexual person, and this final attainment of a predestined 

 position are to many of the phenomena, already described by Weismann 

 in the Hydromedusae ! " Fascinating though the study of the mode of 

 vertebrate development and that of the origin of the embryo had for 

 years been to me, no thought of the cropping-up of any problems of 

 this nature existed, when the investigation of the germ-cells was under- 

 taken. But it was to turn out quite otherwise! 



On the present occasion the writer would gladly have refrained 

 from the enunciation of any theoretical considerations, had not the 

 observed facts revealed such new and peculiar relations of embryo and 

 germ-cells, and had they not shown such significant bearings on the 

 problem of the mode of the development. 



The resemblances to and the differences from the phenomena pre- 

 sented by the Hydromedusae are, however, there, and they have to 

 be reckoned with, or — ignored! 



In the Hydromedusae, as Weismann has demonstrated, the germ- 

 cells usually appear (or arise) at some point or other in the asexual 

 stock, that is, at the bottom in an asexual person or generation. 

 Thence they ultimately migrate into the sexual person, the medusa or 

 its equivalent, that is to say, into the sexual generation. It is im- 

 portant to note, that, with the exception of a few doubtful and not 

 well-understood cases, such as Hydra and Cordylophora^ the sexual 

 person, better generation, never totally disappears, no matter how great 

 be its degeneration. 



In this respect the reproduction and life-history of many a 

 Hydrozoon resemble those of the highest plants. For in the latter 

 also the sexual generation, though it may be reduced almost to nothing- 

 ness, is still invariably represented by something in addition to the 

 germ-cells. 



In the Metazoa, on the other hand, as represented by the skate, 

 the original germ-cells arise outside the embryo, and, indeed, before 

 there is any embryo. If they be merely cells of the latter, set aside 

 for future generations, as is generally held, why are they not in their 



Zool. Jahrb. XVI. Abth. f. Morph. 45 



