92 ANATOMY OF THE RABBIT. 
with the lymphatic trunks form extensive plexuses, in connection 
with which the lymph nodes are distributed. 
THE URINOGENITAL SYSTEM. 
The urinogenital system comprises two primary systems— 
reproductive and urinary—differing widely in their central organs, 
but associated to a certain extent by having common ducts. In 
the rabbit, as indicated in the accompanying diagram (Fig. 48), 
this association extends only to the presence in the two sexes of a 
urinogenital canal, or urinogenital sinus connecting both urinary 
and genital structures with the outside of the body. This canal is 
designated in the male as the urethra, but in the female as the 
ov 

Fic. 49. The principal stages in specialization of the female urinogenital 
ducts in vertebrates. A, frog; B, monotreme; C, marsupial. bl, bladder; 
cl, cloaca; k, kidney; od, oviduct; ov, ovary; r, rectum* u, ureter; us, urinogen- 
ital sinus (vestibulum); ut, uterine tube; v, vagina. Chiefly from figures of 
Gegenbaur and Wiedersheim. 
vestibulum, since the structure known from the human relation 
as the female urethra is only a urinary canal leading from the . 
bladder, and in man is not associated with the reproductive ducts. 
In primitive vertebrates (Fig. 49), the urinary and genital ducts 
open into the posterior end of the digestive tube, the latter forming 
URINOGENITAL DUCTS in this relation a common canal, the 
IN VERTEBRATES. cloaca. In terrestrial vertebrates, the 
urinary bladder is developed as a ventral 
outgrowth of the digestive tube, and, except in amphibians, both 
sets of ducts undergo a migration from their original position on 
to the wall of its canal, the latter being thus trasfnormed into a 
