ON THE EFFECT OF EXTERNAL CONDITIONS 

 ON THE REPRODUCTION OF DAPHNIA 1 



Dr. J. F. McCLENDON 

 Cornell Medical College 



Since in the great majority of organisms only the 

 germ cells are capable of reproducing the entire indi- 

 vidual, the question as to what differences exist between 

 the germ and body cells, and how they arise, is of gen- 

 eral interest. Therefore any change of conditions which 

 affects the germ cells or their relation to the body cells 

 deserves special study. In Dttphnia external conditions 

 not only affect the relation of the germ cells to the body 

 cells, but they affect the egg cells in such a manner as to 

 determine whether they do or do not need fertilization. 

 The purpose of the present paper is not merely to add 

 the results of my experiments to those of other investi- 

 gators, but to tentatively arrange the available data 

 under a general working hypothesis in the hope that 

 some more direct method of investigating the relation 

 of the germ and body cells be devised. 



Last spring (March 10) I began experiments on the 

 effects of environment on Daphnia pidex, De Geer, with- 

 out knowing that AVoltereck was working on the same 

 line. The material came from a small pool investigated 

 by Dr. W. C. Curtis and containing a single strain of 

 this and no other species of Daphnia. For the first few 

 weeks some ice remained on the pool and the tempera- 

 ture did not much exceed 4° C. ; after this it rose steadily 

 to about 20° by the end of May. Specimens from the 

 pool were examined at intervals as a control on the ex- 



