UROGENITAL SYSTEM. 337 
of the development of the young is passed inside the body of the 
-- mother. 
In their early stages the gonads arise anteriorly to the permanent 
kidneys and they retain this position in the adult monotremes (fig. 
337). In all others they are gradually carried farther posterior in the 
abdominal cavity, so that they lie on the caudal side of the kidneys. 
Fic. 337.—Urogenital organs of male Ornithorhynchus, after Gegenbaur. 6, bladder; 
ep, epididymis; k, kidney opened, showing ends of collecting tubules; sr, adrenal; sug 
urogenital sinus; ¢, testis; wr, ureter; vd, vas deferens. 
This transfer of position is effected by a rather complicated apparatus, 
only the broader features of which can be outlined here. In the early 
stages the membranes supporting the gonads (mesorchia, mesoaria) 
are attached to the medial side of the double fold of the serous mem- 
brane around the mesonephros. When the latter organ degenerates 
the fold becomes the broad ligament of the female, while another 
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