SEX RATIOS IX FCETAL CATTLE. 267 



the lengths of the individuals on the abscissa. The number of 

 individuals dealt with in each case is given below the length. 

 From 30 cm. and upward the highest point is 125.96 in the 0-40 

 cm. group, and the lowest is 120 in the 0-30 cm. group. The 

 average sex ratio from 30-100 cm. is 123.27 as indicated by the 

 line AB and the average deviation in this group is therefore 2.88. 



If the line for all individuals over certain lengths is plotted in 

 conjunction with those below the same lengths, the relative sex 

 ratios for the groups compared can be seen. This is done in 

 Fig. i, the values being taken from Table III. From this 

 figure it will be seen that on the average the dotted line runs 

 below the heavy line, the former representing the individuals 

 above the lengths indicated and the latter being those below the 

 same lengths. The exceptions to this are in the 20-30 cm. group 

 and the 80-100 cm. group w r ith a sex ratio of 141, which includes 

 only 70 individuals. 



Therefore it would seem that in the younger stages there may 

 be a slight difference in the viability of the two sexes, the male 

 being somewhat more susceptible to intrauterine disturbances. 

 However, this difference does not seem to be constant enough to 

 be of real significance and one could not, from the data given 

 here, conclude with certainty that in the foetal development in 

 cattle there is a greater mortality in the males during any par- 

 ticular developmental stage. A larger collection of individuals 

 within each group would no doubt clarify the situation, although 

 with all groups considered together, the sex ratio of 123 is 

 probably very near to the true value. 



DISCUSSION. 



A. Reasons for Discrepancies Between the Primary and Secondary 



Sex Ratios in Cattle 



i. A Possible Differential Viability. The only data for the 

 secondary sex ratio with which we can compare the ratios during 

 foetal development are those of Wilckens ('87) who places the 

 secondary sex ratio at 107.3; Pearl and Parshley ('13) who find 

 it to be 113.3 in a population of 480, and Pearl ('17) who finds 

 it to be 100.12 in a population of 1,313 individuals. 



