SEX DETERMINATION IN CATTLE. 2IQ 



totals give approximately a I : I ratio, as on a priori grounds 

 they should. 



Granting the fact that the sex ratio in cattle does change as 

 the time of service changes, what is the biological basis of the 

 phenomenon? Thury's idea was that the sex of the offspring 

 was determined by the relative staleness of the ova. In making 

 this hypothesis it may be assumed either that ovulation occurs 

 early in oestrus entirely, or as a more or less regular process 

 throughout the cestrous period. Unfortunately no exact and 

 thorough studies have been made on the physiology of reproduc- 

 tion in the cow. As has been pointed out by Marshall (loc. tit., 

 p. 316) the fact that artificial insemination has been successfully 

 practiced with cows would indicate that ovulation is independent 

 of coition. In a later paragraph that author states that: " There 

 can be little doubt that in the great majority of mammals 

 ovulation, as a general rule, occurs regularly during cestrus." 

 This would certainly seem to be the case in the cow, since our 

 statistics show that pregnancy may follow coitus at any time 

 during the cestrous period. It has lately been contended by 

 Jentsch, 1 on rather meager evidence, that cows even when 

 served out of the cestrous period will in some cases become preg- 

 nant. The duration of life of the unfertilized egg in the uterus 

 is not known for the cow. It has recently been shown by Lewis 2 

 to be distinctly brief in the case of the sow. He states as the 

 result of numerous observations and experiments that the ovum 

 "does not retain its vitality for more than a few hours after being 

 liberated from the Graafian follicle." But if in the cow ovula- 

 tion begins at or very near the beginning of cestrus, as seems 

 certain from the returns in some of our schedules, where preg- 

 nancy followed a coitus within one or two hours of the beginning 

 of cestrus, then it can fairly be considered as probable that the 

 average age of fertilized ova following coitus late in heat will be 

 somewhat greater than that of the ova fertilized early in heat. 

 Nothing more than an average difference can be claimed (i. e., 

 not every individual ovum fertilized late in heat will be older 



1 Jentsch, A., "Ueber Befruchtung ausserhalb der Brunstzeit beim Rind," 

 Jahrb. iviss. prakt. Tierzucht., Bd. 6, pp. 441-444, 1911. 



2 Lewis, L. L., "The Vitality of Reproductive Cells," Oklahoma Agr. Expt. Sta. 

 Bulletin 96, pp. 147, 1911. 



