12 Sex- Limited Inlieritanee in Cats 



and attractive as this hypothesis is, it cannot be regarded as established 

 until certain contradictory facts have been explained. Apart from the 

 fact that an odd chromosome has been described by Guyer in the 

 spermatogenesis of the FowP, although in this case it is the female and 

 not the male which shows sex-limited transmission, the hypothesis has 

 to face two chief difficulties. The first and less important is that no 

 case is certainly known in which an odd or unequally paired chromosome 

 exists in the female^. This difficulty is not serious, since it is quite 

 possible that a physiological difference may exist between the members 

 of a pair of chromosomes, even though it may not be made visible by 

 our rather crude cytological methods. The second difficulty is more 

 fundamental, and arises from the fact that in the Canary and Pigeon at 

 least, and possibly in other cases, the sex-limitation of the transmission 

 of the characters studied is not absolute. A dark-eyed female Canary 

 does not transmit the factor for the dark eye only to her sons ; for when 

 mated to a red-eyed cock a small proportion of the female offspring are 

 also dark-eyed ^ In the case of sex-limited transmission by the male 

 no certain evidence of exceptions of this kind has been published, 

 although some of Morgan's exceptional cases in Drosophila are most 

 easily explained on the assumption of a failure of the normal sex-limited 

 transmission^. 



The position is therefore this. In one order of Insects and one class 

 of Vertebrates it appears that sex is determined by the egg; in a 

 second order of Insects and a second class of Vertebrates the same kind 

 of evidence suggests that sex must be determined by the spermatozoon. 

 In the second group the evidence for sex-determination by the sperma- 

 tozoon is supported by the visible existence of unpaired sex-chromosomes, 

 giving rise to two kinds of spermatozoa, and yet in both Drosophila and 

 Man there are indications that sex-determination is dependent on or 

 influenced by the mother. Secondly, it appears that in some cases at 

 least sex-limitation in the transmission of characters is not absolute, 

 but is partial as in the case of gametic coupling of other characters, 



^ Anat. Anzeiger, Vol. xxxiv. p. 573. 



2 Baltzer's description of such dimorphism among the eggs of Echinoids can hardly 

 be regarded as proved, until the discrepancies between his results and those of Tennent 

 have been removed. [J. Seiler, Zool. Anz. xli. p. 246, has just published a preliminary 

 note indicating possible chromosome-dimorphism in the eggs of the Motli Phragmatohia. 

 Mar. 1913.] 



=* Durham and Marryat, Reports to Evolution Committee, Royal Soc. iv. 1908, p. 57. 



^ E.g. the occurrence of a miniature ? in Fi from rud.-min. ? x long male. Zeitschr. 

 f. indukt. Abstamm. vii. 1912, p. 333. 



