414 



THE EVOLUTION THEOBY 



is easily seen, for the principal polyp is present earlier than those of 

 the secondary branches, and these again earlier than the polyp which 

 bears the sexual buds, and this, finally, earlier than the sexual bud 

 which it bears. Thus this shunting backwards of the birthplace of 



Fig. 95. Diagram to illustrate the migration of the germ-cells in hydro- 

 medusse from their remotely shunted place of origin to their primitive place 

 of origin in the gonophore, in which they attain to maturity. The state of 

 affairs in Eudendrium is taken as the basis of the diagram. UP, one of the 

 principal polyps, mw, mouth, ma, gut-cavity. *, tentacle. Sia, its stem. A, a. 

 branch of the polyp colony. SP, lateral polyp. Gph, a medusoid-bud completely 

 degenerated into a mere gonophore. Ei, ovum. GH, gastric cavity st, 

 supportmg lamella. The originative area of the germ-cells lies in the stem 

 of the principal polyp at kz"", whence the germ-cells first migrate into the 

 endodei-m of the branch {A) at kz'", creeping within which they reach kz" in 

 thelateral polyp (blastostyle), finally reaching the gonophore (kz) and passing 

 again into the ectoderm. Drawn from my sketch by Dr. Petrunkewitsch. 



the germ-cells means an earlier origin of the primordium {Anlage) 

 of the germ-cells, and consequently an earlier maturing of these. 



But none of all these germ-cells come to maturity in the birth- 

 place to which they have been shifted, for they migrate independently 

 from it to the place at which they primitively arose, namely, into the 



