90 



GERM-CELL CYCLE IN ANIMALS 



&H 



Fig. 29. — Diagram to illustrate the phylogenetic 

 shifting back of the origins of the germ cells in 

 medusoids and hydroids. A composite picture. 

 A, branch of a polyp-colony; P, poljiJ-head 

 with mouth (m) and tentacles ; »S^, stalk of the 

 polyp ; M, medusoid-bud with the bell (Gl) ; 

 T, marginal tentacle; m, mouth; Mst, ma- 

 nubrium ; GphK, a gonoph ore-bud ; GH, gas- 

 tric cavity ; ekt, ectoderm ; ent, endoderm ; 

 st, supporting lamella. The germ cells (kz) 

 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 (kz^), or 

 further down in the stalk of the polj^) at kz'^ 

 — third phyletic stage — or in the ectoderm of 

 the branch from which the polj'p has arisen, 

 at kz'" — fourth phyletic stage of the shunting 

 of the originative area of the germ cells. In 

 the last two cases the germ cells migrate until 

 they reach their primitive place of origination 

 in the medusoid, or in the corresponding layer 

 of the medusoid gonophore, as may be more 

 clearly seen in Fig. 30. (After Weismann, 1904-) 



of these cells in 

 younger em- 

 bryos than yet 

 recorded, and 

 might even dis- 

 close charac- 

 teristics which 

 would enable 

 us to trace the 

 keimbahn in 

 some species 

 back into the 

 early cleavage 

 stages. 



In discussing 

 the germ cells 

 of coelenterates, 

 it is necessary 

 to refer to the 

 work of Weis- 

 mann who has 

 added so much 

 to our knowl- 

 edge of this 

 subject. Weis- 

 mann's position 

 may best be 

 presented in his 

 own words {The 

 Evolution The- 

 ory, Vol. I, pp. 

 413-415, 1904). 



