PORIFERA, CQELENTERATA, VERTEBRATA 85 
By far the most important question arising from 
a study of the origin of the germ cells of Hydra is 
whether these cells arise from ordinary interstitial 
cells, as is clatmed by most investigators, or whether 
they originate from cells that are set aside for re- 
productive purposes at some stage of development, 
as Downing maintains. If the latter be true, “the 
germ-plasm is then continuous in Hydra’ (Downing, 
1905, p. 413). 
Wager (1909) thinks the presence of special prop- 
agation cells to be “extremely improbable”? and 
Tannreuther (1909) does not believe the known 
facts warrant the view that there is continuity 
of the germ-plasm in Hydra. This is, of course, a 
matter that may never be decided definitely, and at 
least not until some method of distinguishing 
the primordial germ cells, if these be present, from 
ordinary interstitial and other cells, has been found. 
Furthermore, if the germ-plasm is continuous, primor- 
dial germ cells must be present in buds, in adults at 
all times of the year, and in pieces of tissue that are 
capable of regenerating sexually reproductive adults. 
That such primordial germ cells exist seems to me to 
be quite possible. 
Hyprozoa. Many Hyprozoa besides Hydra have 
furnished material for germ-cell studies. Thus 
Weismann (1883) reported upon about forty species 
belonging to a number of different families. The 
results of the researches of the various investigators 
do not agree in many instances. In order to indicate 
the variety of the opinions expressed, the data re- 
