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THE AMERICAN NATURALIST [Vol. XLIV 



small odd chromosome previously noted by Herla and later also 

 by Boveri. This peculiar chromosome is said to be very rare in 

 the variety bivalens. Its representative was never discernible 

 in the oogonia and only very rarely in the spermatogonia. From 

 observations of bastard eggs the conclusion is drawn "that the 

 small chromosome surely comes from the spermatozoon in some 

 cases, and possibly from the egg in others" (p. 125 — one instance 

 noted) . The possibility is suggested that the small chromosome 

 is a sex-determining heterochromosome ; but Miss Boring has 

 no doubt also that it is sometimes due to fragmentation. 



Boveri 8 inclines to regard this fifth ("X") — chromosome of 

 Ascaris megalocephala bivalens (present in approximately half 

 the eggs) as comparable to the accessory chromosome of Pyrrho- 

 coris. He considers it more probable that its occasional appar- 

 ent absence (common condition in univalens) is the result of 

 fusion with one of the other chromosomes, than that its presence 

 is the result of fragmentation. He concludes that the "x-chro- 

 mosome" appears to be an independent structure specific for the 

 male sex — as was originally believed by McClung for insects. 

 In analogy with conditions in insects (e. g., Protenor), Boveri 

 assumes a dimorphism of spermatozoa, due to the presence of 

 the x-element in half of the sperm and its absence in the other 

 half. But the unfertilized eggs are supposed to have no homolo- 

 gous elements (there being no evidence of such structures;) ; 

 hence, contrary to his recent generalization that the female fer- 

 tilized egg contained the greater amount of chromatin, the male 

 sex here seems conditioned by, or concomitant with, a relative 

 preponderance of chromatin. This contradiction is elucidated 

 by facts discovered by Boveri and Gulick {op. ext., p. 136) in 

 a study of the spermatogenesis and maturation of Hcterakis, a 

 nematode of the pheasant. The spermatogonial cells have 9 chro- 

 mosomes ; the maturation spindles of the eggs 5. Two types of 

 spermatids are produced, one with 4 the other with 5 chromo- 

 somes; and one kind of egg with 5 chromosomes. The condition 

 is identical with that described by Wilson for the hemipter, 

 Protenor. 



These facts established for Heterakis, give a clue to the more 

 probable state of affairs in Ascaris. Boveri reconciles the dis- 

 crepancies by assuming a close union in the oocytes of the homo- 

 logues of the accessory chromosome with the other chromosomes 

 thus masking their presence. In reality, then, Ascaris mega- 



» Boveri, Th., "Ueber ' Geschlechtschromosomen ' bei Nematoden, ; ' 

 Arch. f. Zellforschung., Bd. 4, H. 1, 1909. 



