218 Basis of Sex Determination 



sexual characters of the other sex make their appear- 

 ance. The most common case is that certain secondary 

 male characters appear in the old female (exceptionally 

 also in the young female with abnormal ovaries) 

 (arrhenoidy). Thus old female pheasants assume the 

 plumage of the male, and in the human female after 

 the menopause and especially among sterile women a 

 beard may begin to grow. The opposite phenomenon, 

 the old male assuming female characters, is not so 

 common. Very interesting observations on changes 

 in the plumage of castrated fowl have recently been 

 made by Goodale. x 



It had long been observed by cattle breeders that in 

 the case of twins of different sex the female — the 

 so-called free-martin — is usually sterile. F. Lillie 1 has 

 recently discovered the cause of this interesting 

 phenomenon. Such twins originate from two different 

 eggs since the mother has two corpora lutea, one in 

 each ovary. In normal single pregnancies in cattle 

 there is never more than one corpus luteum present. 

 The two eggs begin to develop separately in each horn 

 of the uterus. 



The rapidly elongating ova meet and fuse in the small 

 body of the uterus at some time between the 10 mm. and 

 the 20 mm. stage. The blood-vessels from each side then 

 anastomose in the connecting part of the chorion; a par- 

 ticularly wide arterial anastomosis develops, so that either 



1 Goodale, H. D., Biol. Bull., 1916, xxx., 286. 



' Lillie, F., Science, 1916, xliii., 611. 



