SEX DETERMINATION IN CATTLE. 215 



Obviously there arc no differences in age distribution of 

 sufficient magnitude to cause the observed differences in sex- 

 ratios among the three groups. The males used in the three 

 groups are of very nearly the same mean age. The same is 

 true to an even more marked degree of the females of the three 

 groups. It would be difficult to collect a more homogeneous 

 lot of statistics than the present one, so far as concerns age 

 distribution of the individuals in the several samples. 



We have not been able to discover any other factor which 

 could account for the observed differences in the sex-ratios 

 described above. By the most careful tests which we have been 

 able to apply the material appears to be statistically homogeneous 

 except in respect to the time of service. 



Some of the individual records are of interest. The following 

 schedule from Mr. C. E. Clifford may be cited in particular. 

 He was a breeder of registered Jersey cattle and carefully fol- 

 lowed a regular system of service in relation to time of heat. 

 This method in his own words was as follows: "Cover cow twice 

 immediately upon disccvery of heat and keep her separate from 

 all [other cattle] until heat is entirely passed." The sex distribu- 

 tion for 14 calves born in the year for which the return was made 

 was 2 cf cf and 1299. 



DISCUSSION. 



It will have been noted that the results set forth above sup- 

 port, in essential features, the theory of Thury (loc. cit.} as 

 to sex determination in cattle. Thury's original paper was 

 published in French under the title "Memoir sur la loi de produc- 

 tion des sexes chez les plantes, les animaux et rhomme." It 

 immediately attracted the attention of biologists and was crit- 

 ically reviewed and discussed in the Zeitschrift fur wissenschaft- 

 liche Zoologie (Bd. XIII., Heft 4) by Pagenstecher. He later 

 translated Thury's paper into German, added appendices, etc., 

 and it is this German translation which has chiefly been cited 

 by later workers. The essential features of Thury's theory, so 

 far as concerns mammals normally bearing a single young at 

 birth, were set forth in the following words (Thury, loc. cit., p. 16). 



"Das Geschlecht hangt ab vom Grade der Reifung des Eies 

 im Augenblicke, wo es von der Befruchtung getroffen wird. 



