78 



BIOLOGIC BASIS OF SEX 



Fig. 2.1. Twin calves removed fiom the uterus, showing chorionic fusion and anastomosis 

 between major vessels; male twin, left; freemartin, right. (After F. R. Lillie, J. Exper. Zool., 

 23, 371-452, 1917). 



observed and the extent of the vascular 

 union, and also with the stage of develop- 

 ment at which the anastomosis was pre- 

 sumably established. The constancy with 

 which these conditions appear pointed in- 

 evitably to the conclusion that the abnor- 

 mal development of the female twin is 

 caused by the transfusion, from an early 

 stage of development, of a hormone pro- 

 duced by the gonads of the male twin. The 

 invariable dominance of the male member 

 of the pair was explained provisionally on 

 the basis of histologic studies (Lillie and 

 Bascom, 1922; Bascom, 1923) which indi- 

 cated that the testis is active endocrinologi- 

 cally long before the ovary (rf. Bouin and 

 Ancel, 1903). This conclusion has been sup- 

 ported in recent years by the results of 

 castration in mamnialian embryos (q.i\).- 



■Reccnll_\' eNidciice lias come from (|uitc (hl- 

 ferent sources that in most twins or muUiplc biilhs 

 in cattle, placental anastomoses are established at 

 an early stage. Calves, even when of different sex 

 and with other characteristics indicating dizygotic 

 or polyzygotic origin, possess identical comple- 

 ments of l)lood factors (red cell agglutination 

 types), which can only l)e explained on the basis 

 of an early exchange of blood (Owen, Davis and 

 Morgan, 1946). Since erythrocytes are compara- 

 tively short-lived cells, it is indicated in these cases 

 that in-imitive erythroblasts must have been ex- 

 changed early in development and colonized the 

 hemopoietic tissues of the recipients. It has been 

 shown also (Anderson, Billingham, Lampson and 

 Medawar, 1951) that diz3^gotic twin calves of dif- 



II. Methods of Experimental Analysis 



The demonstration in tlie case of the free- 

 martin of the probable nature of the trans- 

 forming agent and its mode of transmission 

 at once suggested means of attacking the 

 problem of embryonic sex differentiation ex- 

 perimentally. At first grafting methods were 

 mainly employed, using the embryos of 

 birds and amjihibians. 



A. CRAFTING OF GONADS OR GONAD TISSUES 

 IX BIRD EMBRYOS 



Historically, the first experiments were 

 those of JNIinoura (1921) who trans})lanted 

 pieces of adult testis or ovary to the chorio- 

 allantois of chick embryos during the period 

 of sex differentiation. Such grafts become 

 vascularized and are then in communication 

 witli the host embryo by way of the umbili- 

 cal circulation. Various modifications of 



ferent sex are, with few exceptions, tolerant to 

 grafts of each other's skin. Tliis is true of skin ex- 

 changes in monozygotic twins (as would be ex- 

 jiected) but is never found in other degrees of 

 r(>hitionship. As in tlie preceding case, the ex- 

 ])lanalion is found in an early transfusion of blood 

 l)etwe(ni the twins. The exceptional cases are 

 doubtless to be explained (as in the freemartin 

 study) on the occasional failure of placental anas- 

 tomosis to occur. This evidence is cited for its 

 bearing on tlic point of early exchange of blood; 

 tlie fact that blood cells and other elements are 

 exchanged does not seem to be of significance for 

 the sex hormone theory. 



