XII.] CONJUGATION AND SEXUAL REPRODUCTION. l6l 



generations, the combination of idants in the different germ- 

 cells of one and the same mother can vary. We can therefore 

 attribute even to parthenogenetic species a certain power of 

 varying, although not to anything like the same extent as in 

 bisexual species. 



By the year 1884 I had commenced a series of experiments 

 to decide the question of variability in purely parthenogenetic 

 species. These experiments are still being carried on, and I hope 

 that I may ultimately be able to make a more complete com- 

 munication upon the subject. I chose for the purpose a species 

 of Cypris (Ostracoda), which was characterized by striking and 

 easily seen markings on the shell. I had at my disposal two 

 very differently marked varieties of the species in question 

 {Cypris reptans), which had been found in the natural state. 

 The species appears to be purely parthenogenetic in this 

 locality ; at any rate I have never found a male, nor a female 

 with spermatozoa in the receptaculum seminis \ The latter fact 

 conclusively proves the complete absence of males ; for in 

 colonies of those species of Cypris which possess males, we 

 always find the receptacula seminis of mature females filled with 

 spermatozoa. Even if it were a mere coincidence that of 

 the many hundreds of individuals examined, all proved to be 

 females, the presence of spermatozoa in their receptacula would 

 still have shown the presence of males, if any had existed in 

 the locality. But the receptacula were, without exception, empty, 

 at all times of the year, and under all the external conditions 

 which obtained during my investigation of the colony. 



My two sub-species are distinguished as follows (see Fig. X) : 

 variety A is lighter in colour, and there are only a few dark 

 green spots of small size on the clay yellow ground-colour of 

 the shell. Variety B appears dark green because the spots 

 are so much larger that they expose only a little of the clay 

 yellow ground-colour of the shell. In both varieties the spots 

 agree precisely as to number and position ; the difference 

 between them is entirely quantitative, but it is considerable, so 

 that the lighter A can be distinguished from the darker B with 

 the naked eye at the first glance. 



The experiment was conducted in the following way: I 



^ Compare my earlier paper ' Parthenogenese bei den Ostracoden;* 

 Zool. Anzeiger, Bd, III. p. 81, 1880. See also Vol. I. p. 301, note 2. 



VOL. II. M 



