354 ABSTRACTS OF PAPERS. [VOL. II. 



large intestines. In Didelphys and other marsupials the pos- 

 terior mesenteric artery is not present. Also in DidelpJiys and 

 other marsupials the internal spermatic arteries are given off 

 from the posterior division of the aorta, and at a point which 

 coincides with the point of origin of the posterior mesenteric 

 artery in other mammals. 



In an adult DidelpJiys killed during the breeding season the 

 writer found present two pairs of functional internal spermatic 

 arteries. The anterior pair was given off from the aorta about 

 midway between the renal and posterior pair of internal sper- 

 matic arteries. The posterior pair, the so-called internal sper- 

 matic arteries of marsupials, was given off from the aorta in 

 the usual manner, as mentioned above. More recently the 

 writer has found another adult female DidelpJiys, in which, in 

 addition to two pairs of internal spermatic arteries, a large 

 posterior mesenteric artery was present. 



In this individual the posterior mesenteric artery arose 

 from the aorta as a single vessel, and at a point which coin- 

 cided with the origin of this vessel in other mammals. On 

 arising from the aorta the vessel passed ventrad through a 

 foramen in the vena cava, and was distributed to the large 

 intestine. The anterior mesenteric artery in this individual 

 supplied the small intestine and the proximal portion of the 

 large intestine. 



The relations of the spermatic arteries were as follows : 



The anterior pair of internal spermatic arteries arose from 

 the aorta, and was distributed to the ovaries as in the above- 

 mentioned case. These arteries appear to be the homologues 

 of those spermatic arteries which in many other mammals 

 arise from the aorta about midway between the renal and 

 posterior mesenteric arteries. The posterior pair of internal 

 spermatic arteries in this opossum were branches of the posterior 

 mesenteric artery, and were given off from this vessel near its 

 point of origin at the aorta. 



It appears to the writer as though in the marsupials, as the 

 result of an arrested development of the original internal 

 spermatic and the posterior mesenteric arteries, a new collat- 

 eral circulation has been established to the genital organs 



