2 GEORGE T. HARGITT 



has been omitted from consideration because of a failure to secure 

 adequate material, but considerable numbers of the other two 

 classes have been carefully investigated. 



The earlier studies (G. T. Hargitt, '09 to '18) presented the 

 data obtained from the investigation of particular species, with 

 some discussion of a general sort in interpreting these observa- 

 tions. It is now necessary to consider the observations in the 

 light of the accumulated knowledge of the various species and 

 to correlate the data obtained from different species. It will also 

 be well to discuss the results of these studies in connection with 

 observations upon the germ cells of other phyla. Certain phases 

 of the problem have been reinvestigated and new data obtained; 

 the results of this new study will be considered in place under the 

 appropriate headings. 



11. ORIGIN OF THE GERM CELLS 

 1 . Place of origin 



The generalization was made many years ago that the germ 

 cells of Hydrozoa always arose from the ectoderm, while in 

 Scyphozoa it was the entoderm which gave rise to germ cells. 

 So far as my observations go, the latter statement is confirmed, 

 but the former is not correct. The genera and species of Hydrozoa 

 which have been investigated are sufficient in number to show 

 that neither the ectoderm nor the entoderm may be considered 

 as the characteristic place of germ-cell origin; on the contrary, 

 these cells may arise sometimes from one layer and sometimes 

 from the other, even in the same species. 



A survey of the available literature of recent years on the 

 germ cells of Hydrozoa gave the following results : All who have 

 worked upon Hydra agree upon the ectodermal origin of the germ 

 cells. Thirteen authors record twenty-three other species of 

 Hydrozoa as producing germ cells in the ectoderm and nine 

 authors record thirty-one species in which the germ cells arise in 

 the entoderm. In many cases from two to four authors have 

 studied the same species, in other cases only a single study has 

 been made of a species. The summary made above includes 



