Two embryos (twins) begin to develop in separate membranes or chorions. At an early stage in this development, however, the arteries and veins of the two become connected, so that the blood of each may circulate through the body of the other. "If both are males or both are females no harm results from this...," since the chemical balance which determines the bodily form in each case is of the same type. But if one is a male and the other a female, the male secretory balance dominates the female in a very peculiar fashion. The female reproductive system is largely suppressed. She even develops certain male organs, and her general bodily appearance is so decidedly masculine that until Dr Lillie worked out the case she had always been supposed to be a non-functional male. She is sterile. The blood transfusion not only alters the sex-type of her body, but it actually modifies the sex glands themselves, so that the ovary resembles a testicle, though dissection proves the contrary. Why does not the female become a true, functional male? Perhaps she does in some cases. Such a one would not be investigated, since there would be no visible peculiarity. In all the cases examined, the embryo had begun its female development and specialization under the influence of a predisposition of the female type in the fertilized egg, before the transfusion began. There is no absolutely convincing mammalian evidence of the complete upset of this predisposition, so all one can say is that it is theoretically possible. Cases of partial reversal, sometimes called "intersexes," are common enough. In birds and insects, where the material is less expensive and experimentation simpler, males have been produced from female-predisposed fertilized eggs and _vice versa_, as we shall see in the next chapter. Dr Bell[2, pp.133f.] points out that the so-called human "hermaphrodites" are simply partial reversals of the sex type from that originally fixed in the fertilized egg. As has been remarked earlier in these pages, there is rarely if ever true hermaphroditism in higher animals--i.e., cases of _two functional sexes_ in the same individual. In fact, the pathological cases in the human species called by that name are probably not capable of reproduction at all.[A]