54 WVMAN REED GREEN". 



A third category of opinion is sometimes tacitly implied by 

 the manner in which the problem of the succession of forms in 

 Cladocera is discussed by those authors who believe that the 

 primary causes are cytological. Were this true the problem 

 would not of course be thereby removed from the domain of 

 possible influence by environmental factors. Here should be 

 mentioned the well-known "kern-plasma" theory of Hertwig. 

 It is supported by Papanicolau, Issakowitsch, Popoff and others. 



The results of Yon Scharfenberg and Papanicolau have been 

 brought by Child (1915) into relation with certain phases of his 

 general theory of organic constitution, as developed in his book, 

 entitled "Senescence and Rejuvenesence." He says (p. 391): 

 "Von Scharfenberg and Papanicolau found that a change in egg 

 character occurred, not only in the course of successive genera- 

 tions, but also in the course of single generations, i.e., the eggs 

 produced early in the life of a female are more likely to develop 

 parthenogenetically into females and those produced later in 

 life into males or to be zygogenic winter eggs. In the earlier 

 generations of a cycle the male producing and zygogenic eggs 

 appear later in the life of the individual, in later generations 

 earlier." I am convinced that the conception that zygogenic 

 eggs normally arise in Daphnians after the parthenogenetic eggs 

 is based on entirely insufficient evidence. Definite statements 

 bearing on this point will be found in Papanicolau's papers 

 (1910, p. 740, and 1911, p. 82). These views are in substantial 

 agreement with those of Issakowitsch (1907) and Scharfenberg 

 (1911), and diametrically opposed to my own since I have never 

 found an instance of the production of ephippial eggs following 

 a period of parthenogenetic reproduction. I am convinced that 

 these conclusions could not have been based upon observations 

 on isolated individual females of Simocephalus vetulus. Gros- 

 venor and Smith (1913, p. 514) working on Mourn rectirostris 

 state: "We did not find any case of a female that had produced 

 eggs parthenogenetically turning into an ephippial female." 



For several years the writer was not able to duplicate the 

 results of those experimenters who claimed that, under certain 

 conditions, reproduction in some Cladocera will proceed for an 

 indefinite number of generations parthenogenetically, and that 



