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COMPARATIVE MORPHOLOGY OF VERTEBRATES 



concerned; the embryonic excretory organs (mesonephroi), and their 

 (Wolffian) ducts; the gonads; the Miillerian ducts; the cloaca, and 

 the anlagen of the external genitalia which arise in the anterior or 

 ventral wall of the urogenital sinus. 



In the early stages the gonads are situated anteriorly to the 

 kidneys. The ovaries are spherical or oval and are equally developed 



Fig. 396. — Urogenital organs of male Ornithorhynchus, after Gegenbaur. b, 

 bladder; ep, epididymis; k, kidney opened, showing ends of collecting tubules; sr, ad- 

 renal; sug, urogenital sinus; /, testis; ur, ureter; vd, vas deferens. 



except in the monotremes where the left is the larger (cf. birds) and 

 it is of interest that eggs — one in Echidna, two in Ornithorhynchus — 

 have been found only in the left oviduct. The ovaries remain near 

 their early position in the monotremes, while in all other mammals 

 they pass farther back, but never outside of the pelvis. They are 

 supported by the mesovaria which are attached to the median side of 

 the double fold of the peritoneum which supports the mesonephros. 



