90 
G 
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, polyp-head 
with mouth (m) and tentacles; St, stalk of the 
polyp; M, medusoid-bud with the bell (Gi); 
T, marginal tentacle; m, mouth; Mst, ma- 
nubrium; GphK, a gonophore-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 
algo attain maturity. In the gonophore-bud 
(GphK) they arise in the ectoderm (kz’), or 
further down in the stalk of the polyp at kz” 
— third phyletic stage — or in the ectoderm of 
the branch from which the polyp 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 medussid gonophore, as may be more 
clearly seen in Fig. 30. (After Weismann, 1904.) 
GERM-CELL CYCLE IN ANIMALS 
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 ccelenterates, 
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). 
