PORIFERA, CQALENTERATA, VERTEBRATA 93 
shifted as far back as to the branch from which 
the polyp has grown out (Fig. 29, A, kz’’’); and 
finally, in the cases in which the medusoid has 
degenerated to a mere brood-sac (Fig. 30, ph), 
even to the generation of polyps immediately 
before, that is, into the polyp-stem from which the 
branch arises that bears the polyps producing 
the gonophore-bud (Fig. 30, kz’’’). Then we find 
the birthplace of the germ-cells still further back 
(Fig. 30, kz’’’’), for the egg and sperm cells arise 
in the stem of the principal polyps (the main stem 
of the colony). The advantage of this arrangement 
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 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 these germ-cells come to maturity in 
the birthplace to which they have been shifted, 
for they migrate independently from it to the place 
at which they primitively arose, namely, into the 
manubrium of the medusoid, which is still present 
even when great degeneration has occurred, or even — 
in the most extreme cases of degeneration — into the 
ectoderm of the brood-sac. This is the case in the 
genus Eudendrium, of which Fig. 30 gives a diagram- 
matic representation. 
“The most interesting feature of this migration of 
