62 FRANK R. LILLIE. 



io-mm. stage of the embryos, and for vascular anastomosis from 

 the 19-mm. stage, or slightly earlier, because the allantois passes 

 completely through the body of the uterus at that time. This 

 antedates the beginning of visible sex differentiation which I esti- 

 mated then at about 25 mm., confirmed since by a much more 

 extensive study. 



These considerations indicated the possibility of exchange of 

 blood between twin embryos before the beginning of sex differenti- 

 ation, but it was not demonstrated that this actually occurred, nor 

 yet how early the blood of the male had any sex hormone con- 

 tained in it. 



The case about to be described (No. 62) gives a fairly definitive 

 answer to these problems. The point of chief interest, demon- 

 strated below, is that the hormones in the male blood have produced 

 a practical inversion of the ovary of the female twin at a stage of 

 3.75 cm., and must hence already have been active for some time. 

 The weight of a male embryo of 3.8 cm. is 3.65 gr. ; the weight of 

 its two gonads is 0.0025 gr., or about one fifteen-hundredth that 

 of the embryo. The quantity of interstitial tissue can be only a 

 small fraction of the total mass of the gonads. Nevertheless the 

 secretion of this relatively minute fraction of tissue diluted by the 

 total blood of both embryos is sufficient during growth of about 

 one centimeter in length to reverse the normal course of develop- 

 ment of the ovary of the female. I know of nothing that gives a 

 more profound impression of the chemical aspect of development 

 than this demonstrable fact. 



Case No. 62: $ 4 cm. greatest length, £ 3.75 cm.; one in each 

 horn of the uterus; collected in February, 191 7. Only one of the 

 maternal ovaries was present and it contained a single corpus 

 luteum. The two chorions were fused so completely that the 

 actual place of fusion can be determined only approximately. In- 

 jections of the umbilical arteries with a starch mass did not pass 

 from one side to the other, but at least one perfectly distinct, 

 though very fine, arterial anastomosis by side branches of the main 

 arteries of the two sides could be demonstrated under the binocular. 



The two embryos were at first diagnosed doubtfully as males 

 and hence received no further attention for a long time, until I 

 had become familiar with definite external characteristics that en- 



