Animal Husbandry Investigations in 1917. 223 



) THE NORMAL LENGTH AND VARIATION OF THE PERIOD OF 

 GESTATION IN CATTLE 



The analyses of the records on the average length of gesta- 

 n in dairy cattle furnish striking evidence of the accuracy of 

 tain human opinions grown up through years of experience. 

 j 1 197 records of cattle births where the length of gestation 

 s known, the average duration of gestation was 281 days, 15 

 furs and 45 minutes, or nearly the customary reckoning of 

 b days. Half of these births had a length of gestation between 

 -.7 days and 286 days. The full range of the length of gesta- 

 ;n for the births was from 215 days to 315 days or a range 

 : 100 days. Since there were no abortions included in the 

 :;a it seems that under normal conditions the length of gesta- 

 :n may have considerable range even though most of the births 

 1: concentrated around one point. 



(c) THE CHANGE IN THE SEX RATIO DUE TO TWIN BIRTHS 



The sex ratio which exists in twins has always been of con- 

 j erable interest both from the point of view of mode of twin 

 eduction and the manner in which identical twins are pro- 

 ved. Our cooperative records furnish us some data on this 

 :?stion which are of great interest. In 2573 births there were 

 :>duced 21 twins or the proportion of twin births is about I 



every 125 births or 8.17 in 1000, compared to a frequency of 

 : inning among women 6 of about 1 in 89. The conclusion fol- 



/s that the frequency of twinning varies considerably with 

 8: species even in those which normally produce only one off- 

 sing at birth. 



Of especial interest is a comparison of the sex ratio as it 

 ::urs in human and cattle births. In the 21 births above men- 

 tried 3 were 2 males, 14 were male and female and 4 were 2 

 fnales. Combining this data with that of Lillie, 7 as they are 



6 Veit, "Beitrage zur geburtshulflichen statistik, "Monatsschr f. Geb. 

 i. VI, 1855. 



7 Lillie, F. R., The Freemartin ; a Study of the Action of Sex Hor- 

 des in the Foetal Life of Cattle. Jour, of Expt. Zoo. Vol. 23, No. 2. 

 P 371-452. 



