UROGENITAL SYSTEM 101 



but relative size, and there are subgeneric or group dif- 

 ferences in the exact shape and size of certain other parts, 

 as Cowper's glands and the corpora cavernosa. It is im- 

 possible, however, to distinguish with certainty which dif- 

 ferences within the material at hand are phylogenetic and 

 which are due to variation in sexual activity. 



The testes in the adult are partly abdominal. The 

 scrotal sac is therefore short and illy defined. The integu- 

 »ment of the cranial portion is covered with short hairs, 

 while caudad it is practically hairless and inclined to be 

 wrinkled. Beneath the integument the testes are covered 

 by the usual diverticula of the abdominal cavity — the 

 tunica vaginalis — which is toughly membranous in charac- 

 ter. They open from the cavity through the inguinal rings, 

 located at the lateral border of either rectus abdominis 

 muscle. This diverticulum is divisible into two layers or 

 lamina — ectalis and entalis — between which are to be 

 found the fibers of the M. cremaster. In young animals 

 the testes are entirely abdominal, but with increasing age 

 the two diverticula gradually develop caudad from the 

 inguinal rings. 



The contents of the diverticulum are suspended from 

 its dorsal wall by a mesenteric membrane, the attachment 

 at the caudal end being stouter. The testis measures 10 

 mm. in length in the inactive Homodontomys and Teonoma, 

 and 17 in Neotoma. From its cranial portion there diverges 

 a bundle of several testicular veins — the pampiniform 

 plexus — which passes within the inguinal ring to join the 

 vena cava. In the drawings this bundle appears as a single 

 vein. Opening from the cranial portion of the testis is 

 also the epididymis, composed of a much-twisted canal, 

 with intermediate tissue. In the animals that are sexually 

 inactive it has shrunken to a mass probably one-fifth the 

 size of the testis, while in the active Neotoma it is fully as 

 large as the latter. The cranial end, or head, located for- 



