48 Anatomy of the Rabbit. 



the ovary, so that the products of the latter, while nominally discharged 

 into the coelom, are actually received directly into the uterine tube. 

 The oviduct itself is phylogenetically the oldest of the urinogenital ducts, 

 since it is identifiable in the embryo as the duct of the pronephros. 



In the rabbit, as in all mammals, the testis is connected with the 

 urethra through the canal formed by the epididymis and ductus 

 deferens, these structures comprising in a modified form a portion of the 

 mesonephros and its duct. Since the excretory functions are fully 

 provided for by the development of a permanent kidney and its duct, 

 the ureter, the ducts of the embryonic kidney have in the adult no urinary 

 connections; but in intermediate vertebrates, in which the mesonephros 

 occurs as an adult kidney, the interesting condition is observable in the 

 male that the duct of this structure serves both urinary and reproductive, 

 functions. 



It will be seen in the rabbit that the female genital ducts, apart 

 from the common urinogenital sinus, include an unpaired portion, the 

 vagina, and a paired portion, comprising the uteri and uterine tubes. 

 In all vertebrates up to and including the monotrematous mammalia 

 the oviducts open separately either into the cloaca, or into the urino- 

 genital sinus. In the higher mammalia, however, a process of fusion is 

 observable, extending from the partial coalescence of the vaginae, as 

 in marsupial mammals, to the complete coalescence of both vaginae 

 and uteri, as in man. Thus, there are recognized in the mammalia 

 the types of (a) unpaired uterus — uterus simplex; (b) semi-divided 

 uterus — uterus bicornis; and (c) completely divided uterus, such as that 

 of the rabbit — uterus duplex. 



In the respective positions of the central organs, as exemplified by the 

 rabbit, there are several features of general significance. Thus, the 

 kidneys (Plate VIII) are paired structures lying on the dorsal wall of 

 the abdominal cavity, where they are interposed between the peritoneum 

 and the dorsal musculature. They are covered by peritoneum only on 

 their ventral surfaces. As indicated above, the permanent kidneys of 

 a mammal are highly specialized structures, but they show in these rela- 

 tions features common to lower forms and dependent in both cases on 

 the primary position of the intermediate mass, from which in the' em- 

 bryo the kidneys are formed. 



The gonads are primarily associated with the dorsal lining of the 

 coelomic cavity, although certain observations on the development of 

 these structures in the lower vertebrates appear to indicate that their 

 elements are assembled from other parts of the embryo. In some of the 

 mammalia, as in all lower vertebrates, the testis occupies an abdominal 

 position in the adult condition ; but usually it undergoes an extensive mi- 

 gration, passing from the abdominal cavity into a special sac of peritoneum 

 enclosed by the scrotum. This change — described as the descent of 

 the testis^is effected through the agency of a muscular cord, the guber- 

 naculum. In many cases, as in man, the cavity enclosing the testis is 

 completely separated from the abdominal cavity, but in the rabbit a 

 more primitive condition is retained in which the sac of the testis is 

 widely open to the abdominal cavity, and the organ thus passes freely 



