228 CLAUDE W. MITCHELL 



Certain general conclusions previously arrived at by the author, 

 which bear upon the question of sex-determination may be briefly 

 stated before proceeding to the actual experiments. The rotifer 

 in question is a tri-morphic if not a poly-morphic species. The 

 form which emerges from the resting egg is preponderatingly 

 parthenogenetic, while the later, larger, and more complex types 

 are copious male producers. This smaller, saccate type may also 

 be reached by degeneration from the others, the humped and 

 campanulate types. It is plainly upon a lower physiological 

 level. 



The study of the species has also disclosed that a marked meta- 

 bolic rhythm runs through long series of generations and mani- 

 fests itself in a rhythm of reproduction, even comprising periods 

 of marked depression as well as acceleration. As stated in a 

 preceding article this rhythm is not the result of external con- 

 ditions. Depressions and accelerations in the reproductive 

 rate manifest themselves strongly when external conditions are as 

 uniform as possible. Nevertheless, it should be stated that 

 this inherent rhythm although relatively independent of external 

 conditions is not absolutely independent of them. A rise in 

 temperature or a fresh nutrititve stimulus may check a tendency 

 towards depression or raise still higher the wave of acceleration. 



As it has been shown there exists a definite relation between 

 the amount of male production and the type of individual, and 

 as it appears that there is also a correlation between the type of 

 individual and physiological factors such as we have mentioned, 



so-called 'sexual female,' whereas these two classes of individuals are sharply dis- 

 tinguished in certain other groups. In contradistinction to this claim we would 

 raise the question whether the exact opposite is not nearer the truth, namely, 

 whether the obvious morphological differences which sometimes accompany these 

 diverse reproductive habits have not led to the over-accentuation of their inher- 

 ent differences. Sex-determination there would be in any case. But the nature, 

 degree, or importance of the phenomenon in this special instance is bound up with 

 the whole question of the relation between parthenogenetic and sexual reproduc- 

 tion. Do not many recent advances point to the fact that the differences between 

 the two have in the past been over-estimated? Work which is nearing comple- 

 tion upon certain problems in the genus Asplanchna point with surprising empha- 

 sis to the fundamental unity between parthenogenetic and sexual reproduction 

 in this species. 



