12 GEORGE T. HARGITT 



3. Conclusions 



In the last twenty years the reports of investigations upon the 

 origin of germ cells of Hydrozoa show more species in which such 

 cells proceed from the entoderm than from the ectoderm. 

 Numerous cases are recorded in which the place of origin differs 

 in the sexes of a single species and where the same individual 

 may produce germ cells from different layers. Furthermore, the 

 germ cells come from different sorts of cells. All of this points 

 to the conclusion that the place of origin is variable and not a 

 matter of any significance. A few cases are reported of the origin 

 of germ cells in the embryos or larvae of Hydrozoa, but new 

 investigation of these gives no confirmation of this. Interstitial 

 cells are differentiated in early ontogeny and undergo early 

 specialization into ganglion, nettling, and other cells, but those 

 not so specialized are alike in all respects and at most persist as 

 somewhat inactive cells. During all the life of the polyps these 

 produce nettling cells, form replacing cells, and, in some species, at 

 sexual maturity produce germ cells. 



In none of the Hydrozoa has the differentiation of germ cells 

 been demonstrated in early ontogeny. On the other hand, 

 observations of several species have demonstrated that germ cells 

 may arise from body cells directly, either by the transformation 

 of an entire cell or from the transformation of one half of such a 

 body cell. Obviously in such cases an early differentiation of 

 germ cells is out of the question, and it is believed to be typical 

 of Hydrozoa to form their germ cells only at the time of sexual 

 maturity. 



III. THE GERM-PLASM THEORY 



1 . General statement and discussion of the theory 



This theory has been much discussed and many weighty 

 objections have been raised against it; at the same time it has 

 been strongly defended and important evidence brought forward 

 to uphold it. Probably the lines of defense, as well as of oppo- 

 sition, are so well known as not to require further review. There- 



