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THE AMEBIC AN NATURALIST [Vol. XLV 



component than does the other. Eggs fertilized by the one sort 

 of sperm develop into females, those fertilized by the other sort 

 develop into males, as is shown by a cytological study of the two 

 sexes, the female invariably containing the greater number of 

 chromosomes or chromosome components. 



Some results of especial interest have been obtained by Boveri 1 

 from studies of a little nematode (Rhdbditts nigrovenosa ) which 

 occurs as a parasite in the lung of the frog. A free-living genera- 

 tion alternates with the parasitic one. It has long been known 

 that the parasitic generation consists exclusively of females, but 

 the free-living one of both sexes. According to Leuckart and 

 Boveri, parthenogenesis may occur in the parasitic generation, 

 though it is not the exclusive method of reproduction in this 

 generation. For Anton Schneider, recently confirmed by Boveri, 

 found spermatozoa in the genital tract of the parasitic female, 

 and further established the remarkable fact that these sperma- 

 tozoa develop in the ovarian tubules of the young female herself, 

 which therefore, though a female in external form, is really a 

 hermaphrodite. The close-fertilized eggs of the parasitic worm 

 develop into embryos which are voided with the feces of the host 

 and form the free-living generation, consisting of sexually sepa- 

 rate males and females. These produce in turn, from fertilized 

 eggs, the parasitic generation composed exclusively of hermaph- 

 roditic females. 



The question which Boveri studied was this — how is sex deter- 

 mined in the parasitic generation Are the spermatozoa of the 

 self- fertilized mother dimorphic, and if so how do they arise? 



First of all he established the fact that the spermatozoa found 

 in the parasitic females are of two sorts. The chromosomes may 

 be counted even in the mature sperm, and were found to be in 

 part of the spermatozoa six in number, in part of them five. 

 In the egg and polar cells were found always six elements. The 

 fertilized egg therefore must contain either twelve or eleven chro- 

 mosomes. From one of the former sort doubtless develops a 

 female, from the latter a male. For in the male of the free-living 

 generation Boveri found 11 chromosomes disposed as follows in 

 the spermatocyte of the first order: 5 tetrads, 1 dyad (x chromo- 



