124 



CARNEGIE INSTITUTION OF WASHINGTON. 



The intergrades are of all grades, ranging from female intergrades 

 with one of the secondary sex-characters slightly male to female inter- 

 grades with all of the secondary sex-characters fully and strongly 

 male. In the Simocephalus sex-intergrade strain reported a year ago 

 there occur in addition to some normal females and many female 

 intergrades: (1) a few hermaphi-odites with various combinations of 

 male and female secondary sex-characters ; (2) some male intergrades 

 with one to several female secondary sex-characters; and (3) a great 

 many normal males. In the new Daphnia longispina sex-intergrade 

 strain, however, no hermaphrodites or male intergrades have so far 

 been observed while normal males are extremely rare in this strain, 

 whereas in the Simocephalus intergrade strain a considerable percent- 

 age of the individuals are normal males. The sex-intergrade strain of 

 Daphnia longispina runs distinctly less toward maleness than does 

 that of Simocephalus. 



In other regards the two strains possess many characters in common. 

 Both strains of sex-intergrades vary in their productivity. The more 

 highly male a female intergi'ade is the less productive she is Ukely to 

 be. Most of the female intergrades which have most of their secondary 

 sex-characters those of a male are sterile. Ovarian eggs are produced 

 and develop almost to the point of being transferred to the brood 

 pouch, but their development then ceases and they disintegrate within 

 the ovary. 



The occurrence of a second sex-intergrade strain adds greatly to the 

 interest and significance of such forms. Sex is again revealed not as 

 a precise and definite state, not as an alternative condition, but as a 

 purely relative condition. 



Non-necessity or sexual reproduction in Cladocera. — In the last 

 several reports I have refeiTed to the significance of the fact that the 

 lines of the various species of Cladocera continued reproducing par- 

 thenogenetically without apparent diminution of vigor. Such is still 

 the case. The oldest lines of different species have now (Septem- 

 ber 1, 1917) been reproducing parthenogenetically in the laboratory 



as follows: 



Table 6. 



