PORIFERA, CCELENTERATA, 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, Gph), 

 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, hz'"). 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 



