PORIFERA, CCELENTERATA, 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 claimed 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. 



HYDROZOA. Many HYDROZOA 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- 



