130 ANATOMY OF THE RABBIT 



produced in much smaller numbers than the spermatozoa. The ova 

 undergo their primary development as single cells in the tissue of 

 the organ, but at times, through rupture of the enclosing follicles, 

 they are released at the surface, and thence pass directly into the 

 open mouth of the uterine tube, the narrow first part of the oviduct. 

 In this tube the ova may come into contact with spermatozoa, 

 fusion with one of which constitutes fertilization of the ovum. 

 (Spermatozoa deposited in the vagina are carried into the uterus 

 by muscular action of the duct, traverse the uterus by their own 

 locomotor activity, and are carried up the uterine tube largely by 

 the cilia lining it.) If fertilized, they begin their segmentation and 

 further development into an ^.^^ ^^ 



embryo, the latter becoming J^ At ^^\\/7^ \ / 

 attached to the wall of the \\// \"/ \"/ 



more posterior part of the Vh^ W \A 



oviduct, which is enlarged to { j '^ 1 1 ^ f ] ^ 



form the uterus. A placental 



Fig. 78. Three stages of specialization of the 



connection IS formed by which uterus, a, uterus duplex; B, uterus bicornis; 



. , . . , 1 C, uterus simplex, t, uterine tube; u, uterus; 



nourishment is carried to the v, vagina. 

 embryo during the period of 



intra-uterine life, in the rabbit about thirty days. The rabbit has 

 two complete uteri, the cavities of which are connected distally 

 with the unpaired vagina, and through this with the urinogenital 

 sinus or vestibulum. The size and appearance of the uteri depend 

 upon the age of the animals examined, and upon whether or not 

 they are pregnant or have borne young. The uteri of pregnant 

 fem.ales are greatly enlarged and vascular. They contain from five 

 to eight young, the position of which may be easily seen from the 

 expansion of those parts of the tubes in which they lie. 



The paired condition of the uteri in the rabbit is especially 

 instructive because of its primitive nature as compared with the 

 arrangement in many other mammals. Paired oviducts (Fig. 69, A) 

 are the rule in lower vertebrates, where the function is simply to 

 carry the eggs to the outside of the body. This condition is retained 

 with minor modifications to the monotreme stage of mammals, 

 but in higher forms of the latter the ducts are progressively coa- 

 lesced. In marsupials the posterior part of each oviduct is differ- 

 entiated as a vagina, which is still paired, while in placentals the 



