412 



THE EVOLUTION THEORY 



animal are fonncd, has been shifted backwards in the course of 

 phylogenetie ovohition, that is, has been moved nearer to the starting- 

 point of development. This shifting has exactly followed the ' germ- 



msmmM . 



ra7r.!JJ.I.I.!.lal.'i!Ji!.!»'Jii .l'ki'»T;ii^i^l«'.!» \i'';'o'.'.l.'.'.'.'.ft'.V. 



GJf 



Fig. 94. Diagram to illustrate the pliylogenetic shifting back of the 

 origins of the germ-cells in medusoids and hydroids. A composite picture. 

 A, branch of a polyj) colony. P, j^olyj^-liead with moiith {m) and tentacles. 

 St, stalk of the polyp. M, medusoid-bud with the bell {Gl). T, marginal tentacle. 

 ??i, mouth. j¥s(!, manubrium. Gi:'7;A', a gonophore-bud. Gif, gastric cavity, ekt, 

 ectoderm, ent, endoderm. st, suppoi-ting lamella. The germ-cells {ks) arise 

 in the medusoid in the ectoderm of the manubrium — first phyletic stage — 

 where they also attain maturity. In the gonophore-bud (GphK) they arise in the 

 ectoderm (A-j", or further down in the stalk of the polyp at A;" — third phyletic 

 stage, or in the ectoderm of the branch from which the polyp has arisen, at 

 kz" — fourth phj-letic stage of the shunting of the originative area of the 

 germ-cells. In the two last cases the germ-cells migrate until they reach their 

 primitive place of origination in the medusoid, or in the corresponding layer 

 of the medusoid gonoj^hore, as may be more clearly seen in Fig. 95. Drawn 

 from my sketch by Dr. Petrunkewitsch. 



tracks,' as we shall see, although in some cases it would have been 

 more advantageous if the birthplace of the germ-cells could have lain 

 outside of these. Obviously, then, it is only the existing cell-genera- 

 tions of the germ-track which were able to give rise to germ-cells, or, 



