SEX RATIOS IN FCETAL CATTLE. 



F. M. JEWELL. 



I. INTRODUCTION. 



It is a matter of common knowledge that among animals there 

 is an approximate equality of numbers between the sexes, and 

 the possible causes for this equality have long been a matter of 

 speculative interest. Since the dimorphic character of the 

 spermatozoa in various animals including cattle (Wodsedalek 

 '20) has been confirmed by many investigators, it is generally 

 conceded that the sex is determined as soon as the successful 

 spermatozoon enters the ovum. 



Therefore, according to the laws of chance, one would expect 

 to find an equality of sexes at the time of conception. However, 

 a vast amount of evidence shows that this is not the case, but 

 that throughout the animal kingdom, with few exceptions, there is 

 a preponderance of males at birth. 



In the collection of twins in foetal cattle used by Dr. F. R. 

 Lillie in his work on the free-martin, the sex ratio was 134 males 

 to 100 females and although the total number was but 108, this 

 represented such a wide departure from the sex ratio at birth, 

 as given by other investigators, as to demand attention. This 

 indicates that there may be either a marked difference in the 

 sex ratios in the f octal and the born population, or that there is 

 some interference in the chance determination of sexes in dizy- 

 gotic twinning in cattle. 



This investigation was undertaken to determine the fcetal 

 sex ratio in cattle in order that this discrepancy might be cleared 

 up, but more particularly to procure data for comparison of the 

 primary, secondary and tertiary sex ratios as stated by A. M. 

 Schultz ('18). 



First it will be necessary to give a statement of the existing 

 data on sex ratios in cattle, expressed as the number of males 

 per 100 females. 



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