202 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



oping from such eggs must be almost invariably female, because males, as 

 already stated, are extremely rare. Yet for the very reason that males 

 are occasionally produced, we are forced to the conclusion that the male 

 character is present, recessive, in the ordinary female of Rhodites. If so. 

 the egg does not eliminate the character of that sex at the formation of 

 the second polar cell, but retains the characters of both sexes, and so has a 

 formula, (J 9, a supposition for which we have warrant in the mosaic 

 gametes of spotted mice. In further support of this idea may be men- 

 tioued the observation of Henking, that in the maturation of the egg of 

 Khodites no reduction diviaion occurs ; the nucleus of the ovarian egg, the 

 three polar nuclei, and the nucleus of the mature egg, all alike contain 

 nine chromosomes each. It is probable, therefore, that normally the 

 second maturation division in Rhodites is qualitatively like the first, an 

 equation division, in whicli no segregation of sex characters takes place. 

 But the occasional production of a male Rhodites indicates tliat the 

 egg still retains a capacity to eliminate the dominant female character in 

 maturation, and so to become male, as do the eggs of other partheno- 

 genetic animals under appropriate conditions. 



B. IItdatina senta. 



Hydatina senta differs from other iiarthenogenetic animals in the fol- 

 lowing respects. Its female summer eggs, instead of forming one polar cell, 

 form none. Its male summer eggs and fecundable (winter) eggs (doubt- 

 less at the outset one and the same sort), instead of forming tivo polar 

 cells, form one. It is evident that one of the normal maturation 

 divisions has in this species been omitted. Clearly it is not the normal 

 second division, for the single one which occurs is a segregation (or 

 reduction) division. Manifestly, then, tlie maturation division which 

 is suppressed in Hydatina is the normal first maturation division of 

 fecundable eggs, the sole maturation division of eggs not fecundable. 



Corroborative evidence of the correctness of this interpretation comes 

 from an unexpected source, the mammals. Sobotta ('99) finds that in 

 the egg of the mouse there occurs usually oidy a single maturation 

 division. Tiiis is the homologue of the setond maturation division of 

 other animals. When two maturation divisions occur in the same egg, 

 the second is always of the same type as the single maturation division of 

 other eggs, and it occurs in a like stage of matuinty of the Graafian 

 follicle. The single maturation division of one type of egg, and the 

 second maturation division of the other type, are apparently alike 

 reduction divisions, for the mitotic spindle, according to Sobotta's figures, 



