342 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
‘duct is also largely lost, the lower portion sometimes persisting as a small blind 
tubule imbedded in the prostate gland and known as the uterus masculinus. 
In the testes between the tubules are smal] aggregates of cells known as inter- 
stitial cells, which have recently been shown to be glands with internal secretion. 
In man their products, which pass into the blood, apparently cause the assumption 
of the secondary male characters—growth of hair on the face, change of voice, etc. 
—at the time of puberty. There would also seem to be some analogous structure 
in the ovary governing the development of female characteristics and controlling 
some of the features of. menstruation. 
There are a number of accessory glands connected with the genital ducts, these 
being usually better developed in the male than in the female. Only the more 
Fic. 341.-—Diagram of male genitalia of beaver, Castor canadensis, after-Weber. a, 
anus; ag, anal gland; }, urinary bladder; gv, gland of vas deferens; oa, opening of anal gland; 
op, os penis; p, prostate; pp, preputial gland; r, rectum; u, ureter; vd, vas deferens. 
prominent are mentioned here. The seminal vesicles (present in some rodents, 
bats, insectivores and in ungulates and primates) are a pair of tubular or saccular 
glands opening into the vasa deferentia just before their entrance into the urogenital 
‘canal. The prostate glands, which occur in all placental mammals with the 
exceptions of edentates and whales, are connected with the urogenital canal. 
Farther along the canal are Cowper’s glands which occur in almost all mammals 
as scattered bodies or aggregated into larger masses, and surrounded by smooth 
muscle. 
Concerning the functions of these glands considerable uncertainty exists. 
From the fact that removal of the prostate and the seminal vesicle in rats prevented 
fertilization, and the further fact that the secretion of the seminal vesicles increases 
the activity of the spermatozoa, it seems probable that they are of great importance 
in connexion with fertilization. Then it has been shown that in some instances 
the coagulation of the secretion of these glands closes the vagina after copulation 
has occurred, thus preventing the exit of the sperm. 
COPULATORY ORGANS. 
In many vertebrates the eggs are fertilized after passing from the 
oviducts. This is the case with the cyclostomes, most fishes, with 
the exception of the elasmobranchs, and with many amphibians. In 
