LIFE CYCLE OF SIMOCEPHALUS VETULUS. 5! 



are different from the offspring of females produced partheno- 

 genetically; whether any difference is to he Ion IK I between the 

 offspring of parthenogenetic females which have passed through 

 a sexual period and (hose \\hich have not; whether a sexual fe- 

 male may not be retained in the sexual state, etc. 



This paper presents data accumulated chiefly during the four 

 years from 1914 to KJIS at ( 'arleton College biological laboratory, 

 Northfield, Minn., bearing upon the solution of these and other 

 problems and definite conclusions regarding some of them. 

 There naturally arises the very interesting and important problem 

 of ascertaining just what correlations exist between the general 

 course and the cytological aspects of the life cycle. The details 

 of my work on this latter phase of the problem are to be given 

 in another paper, now under way. 



II. LITERATURE REVIEW. 



The theory that there is a sex cycle in Cladocera which is 

 independent of external factors was formulated by August 

 Weismann. It is now most certainly known that, although the 

 correlation existing between environmental factors and the pro- 

 duction of parthenogenetic and sexual forms is not exact, certain 

 environmental complexes do exist which completely inhibit the 

 appearance of males and sexual females. 



A review of the very extended work of Weismann (1877-87) 

 and his contemporaries such as Kurz (1875) and Schmankewitsch 

 (1877), and of later writers as Issakowitsch (1907), of Kuttner 

 (1909), Woltereck (1909), Papanicolau (1911) and others, 

 leaves one very unsettled as to what is the general determinative 

 principle underlying the course of events in the normal life 

 cycle of even the well known and much studied genera such as 

 Moina, Daphnia, and Simocephalus. The \Yeismannian con- 

 ception of a relatively fixed "generation cycle" for Cladocera is 

 well known, and has been adopted wholly or in a modified form 

 by practically all writers up to about 1914, although its truth had 

 been questioned by several experimenters. About this time 

 some results were obtained by several workers using various 

 species of the three genera mentioned, which must be interpreted 

 as positive evidence against the theory. Since Weismann V 



