The development of the genital tract seems to be essentially the same through all orders in that the genital ridge forms the ovary and its attachments while the Mullerian ducts supply the tubes, uterus and vagina. Early in fetal life these two longitudinal ducts begin to approach one another, and by the end of the third month should be in the position which they are to retain for the full development of their end result. In the human being this position is complete union and fusion, with the production of a single tube from the uterine fundus to the exterior, while in the lowest mammals, edentates and marsupials, the two Mullerian ducts retain their lateral position, and upon completion of embryonal life a double tube from the ovaries to the exterior is found. If the normal fusion of the ducts does not take place, and if for any reason their proper relation is not reached, an abnormality will result varying according to the stage of development that has been reached. Thus in man instead of a single uterus of triangular shape and a single cervix, a double set of tubes may be found. It can be shown that the abnormalities of the human uterus are of definite and fixed types corresponding to an arrest of fusion or completion of the developmental cycle as given for the Mullerian ducts and further that these very deformities are comparable to normal organs of lower orders. In other words abnormalities in arrest of development in the human uterus represent normal types of lower uteri at various evolutionary stages. In monotremes there are two ovaries, tubes, uteri, cervices, a urogenital passage and a clitoris. This corresponds with Uterus didelphys with a single vagina. This abnormality while occurring in the lowest mammal, does not represent the most marked deformity known for the human being— that in which the double tube remains to the vaginal outlet, a condition found in the marsupials. In this order the uteri are entirely separate, and each is fitted with its own vagina. The next higher order, Edentata, seems somewhat out of place if it be judged by its female genitalia since it is possessed of a triangular uterus and single vagina but without distinct cervical segment; the tract is very similar to that of the Primates.