PORIFERA, CQELENTERATA, VERTEBRATA 89 
One more Hyprozoon may be mentioned — 
Gonothyrea loveni— since Wulfert (1902) traced 
the germ cells of this species back to the planula 
stage where they arise from the interstitial cells of 
the ectoderm and later undergo characteristic 
migrations. 
Our knowledge of the origin of the germ cells 
in other coelenterates is very fragmentary and even 
less decisive than that of the Hyprozoa. For this 
reason a consideration of the subject is omitted here. 
Discussion. As in the Porirera we are here 
confronted with the question whether or not there is 
continuity of the germ-plasm in the C@LENTERATA. 
There is sufficient evidence for the belief that the 
cells which develop into germ cells are not derived 
from the ectoderm or the entoderm but belong to a 
special sort of propagative cells which are scattered 
about among the other cells throughout the body 
and which give rise to ova or spermatozoa under 
certain environmental conditions differing in the 
different species. This conclusion is based partly 
upon the results of Downing (1905, 1908, 1909), 
who still holds, as stated in his published papers, 
that there is continuity of the germ-plasm in Hydra; 
and upon the fact that germ cells have been recog- 
nized in the young hydranths of Clava (Harm, 1902) 
and in the planula of Gonothyrea (Wulfert, 1902). 
It seems certain that more careful studies of the 
early stages of cclenterates with special regard 
to the origin of the germ cells and with the use of 
many and varied stains would result in the discovery 
