92 



GERM-CELL CYCLE IN ANIMALS 



Fig. 30. — Diagram to illustrate the migra- 

 tion of the germ cells in hydromedusae 

 from their remotely shunted place of origin 

 to their primitive place of origin in the 

 gonophore, in which they attain to ma- 

 turity. The state of affairs in Eudendrium 

 is taken as the basis of the diagram, mu, 

 mouth ; ma, gut-cavity ; t, tentacle ; Sta, 

 stem; A, a branch of the polyp-colony; 

 SP, lateral polyp; Gph, a medusoid-bud 

 completely degenerated into a mere gono- 

 phore ; Ei, ovum ; GH, gastric cavity ; 

 st, supporting lamella. The originative 

 area of the germ cells lies in the stem of 

 the principal pob-p at kz"", whence the 

 germ cells first migrate into the endo- 

 derm of the branch (A) at kz'", creeping 

 within which they reach kz" in the lat- 

 eral polyp (blastostyle), finally reaching 

 the gonophore {kz) and passing again 

 into the ectoderm. (^After Weismann, 

 1904.) 



Podocoryne. Al- 

 though perfect 

 medusoids are 

 formed, these 

 have their germ- 

 cells fully devel- 

 oped at the time 

 of their liberation 

 from the hydroid 

 colony. But in 

 species in which 

 the medusoid- 

 buds have really 

 degenerated and 

 are no longer lib- 

 erated, the birth- 

 place of the germ- 

 cells is shifted 

 even farther back^ 

 and in the first 

 place into the 

 stalk (SU hz") of 

 the polyp from 

 the gonophore- 

 buds. This is the 

 case in the genus 

 Hydractinia. In 

 the further course 

 of the process the 

 birthplace of the 

 germ -cells has 



