714 A MANUAL OF VETERINARY PHYSIOLOGY 



ova produced. Where no such selective action occurs in the ovary, 

 the proportion of the sexes of ovarian ova produced is governed 

 by the laws of heredity. 



Twin and Multiple Births. — As a rule, only one embryo results 

 from the fertilisation of a single egg, but occasionally two develop, 

 and in this case the foetal membranes are common to both. 

 Twin births, however, more commonly arise from the fertilisa- 

 tion of two eggs ; in the case of multiple births there is no 

 doubt that each embryo is the result of the impregnation of 

 a separate ovum. 



Twin births resulting from the impregnation of a single egg 

 are found to occur in cattle, and the offspring generally consists 

 of a potent bull calf and so-called heifer calf, the latter being 

 sterile and known as a free-martin. The nature of a free-martin 

 has been a developmental problem for one hundred and fifty years. 

 John Hunter drew attention to it and preserved the parts, 

 which, in the light of present knowledge, have recently been 

 re-examined.* To Berry Hart we are indebted for a solution 

 of the problem. He points out that as in the free-martin two 

 embryos arise from a single egg, the genitals of each have to be 

 provided from a single cell, which normally might give rise to 

 one perfect male. Instead of two perfect males resulting, one 

 perfect and one imperfect follows, for the free-martin, though 

 generally resembling the female externally, is in reality an 

 imperfectly developed male. The external part of the genital 

 tract is like that of the cow, but the inner part is defective, 

 there being a very rudimentary vagina and a knob of uterus, 

 with testes and Wolffian bodies imperfectly developed. Hart, 

 in his monograph, points out that there are female as well as 

 male free-martins, but they are very rare. A free-martin 

 co-twin with a male is a male, a free-martin co-twin with a female 

 is a female. The most frequent variety is the male— viz., a 

 potent bull born co-twin with an apparent heifer which is 

 sterile. In all cases one animal is potent, the other sterile. It 

 has been usual to regard free-martins as a form of hermaphrodism, 

 but embryologists have established that an hermaphrodite must 

 possess both male and female sexual organs, and this is not the 

 case in a free-martin. 



Implantation of the Ovum. — In the sheepf the impregnated egg 

 enters the uterus on the fourth or fifth day, and travels slowly along 

 it until the ninth day. On the ninth day the zona radiata ruptures, 



* ' The Structure of the Reproductive Organs in the Free-Martin,' by 

 D. Berry Hart, M.D., Proceedings of the Royal Society of Edinburgh, 

 vol. xxx., part iii. 



t ' The Morphology of the Ungulate Placenta,' by R. Assheton, M.A., 

 whom, in the above account of the sheep, we have entirely followed 

 {Phil. Trans., B, vol. exeviii., 1905). 



