G. Smith has discovered a fact which shows that chemical changes must underlie these morphological transformations of primary or secondary sexual characters. He noticed that in male crabs the presence of the parasite _Sacculina_ changes the contents of the fatty constituents in the blood, making them equal to that of the female. Vaney and Meignon had previously shown that during the chrysalid stage the female silkworms have always more glycogen and less fat than the males. The castration by parasites is paralleled by what Caullery calls the castration by senility.[191] In certain birds and also in mammals at the time when the sexual glands cease to function certain secondary sexual characters of the other sex make their appearance. 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.[192] [191] Caullery, M., _Les Problèmes de la Sexualité_. Paris, 1913. [192] Goodale, H. D., _Biol. Bull._, 1916, xxx., 286. 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[193] 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. [193] Lillie, F., _Science_, 1916, xliii., 611. 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 particularly wide arterial anastomosis develops, so that either fetus can be injected from the other. The arterial circulation of each also overlaps the venous territory of the other, so that a constant interchange of blood takes place. If both are males or both are females no harm results from this; but _if one is male and the other female, the reproductive system of the female is largely suppressed, and certain male organs even develop in the female. This is unquestionably to be interpreted as a case of hormone action._ The reproductive system of these sterile females is for the most part of the female type, though greatly reduced. The gonad is the part most affected; so much so that most authors have interpreted it as testis.