THE URINOGENITAL SYSTEM 199 
exercised a great influence on the lower portion of the abdominal wall. This 
would appear to have involved the invagination of a more or less circum- 
scribed portion of the lateral abdominal muscles by the glandular apparatus 
(which in the Monotremata has already attained large proportions), leading 
up to the differentiation of a compressor of the mammary organ out of the 
transversus muscle. He further surmises that this, which represented a 
primitive conus inguinalis, was retained in the Marsupials to assist in the 
extra-uterine nourishment of the young, and that it disappeared in the 
Placentalia owing to the substitution of other methods of providing for the 
offspring. The invagination of the conus into the celom must, like the 
maturation of the glandular complex, have occurred periodically. The male 
conus became related to the male genital gland, and the periodic displacement 
of the latter (towards the point of the least resistance) must thus be associ- 
ated with its great increase in size at the times of sexual activity. For 
the ovaries this last factor has not to be taken into account, as they do not 
undergo such great variations of size; and further, their power of descent 
is greatly diminished in consequence of their position in relation to the 
Millerian ducts. 
The essential, that is the first, cause of the descensus remains unexplained, 
and the origin of the ligamentum inguinale is still a complete enigma. On 
the other hand, its connection with the uterus, its periodical increase in size 
during pregnancy, and especially its near relation to the conus inguinalis, and 
thus to the mammary organ, make it very probable that it originally arose 
in the female, and was transferred to the male with the other parts belonging 
to the mammary organ. 
SUPRARENAL BODIES 
These organs are probably to be traced to a double origin, 
partly from the mesonephros and partly from the sympathetic 
_hervous system. Their physiological significance is as_ little 
known as their primitive history, and it is not certain whether, 
so far as Man is concerned, they are phylogenetically in a pro- 
gressive or in a retrogressive condition. 
The latter assumption is the more probable when we consider 
their great development during embryonic life. On the other 
hand, their rich blood-supply indicates some important physio- 
logical function performed throughout life. 
which is to be found on the level of the scrotum in the young stage of all 
Mammals, including Man, and which at a later stage meets the corresponding area 
of the other side in the middle line. The numerous smooth muscles which 
constitute the tunica dartos appear to correspond with the smooth muscle layer 
_of the glandular area in the Monotremata. In all Mammals the area scroti is 
distinguished by the fact that the hair grows on wart-like elevations which 
are closely crowded together—a peculiarity which gives the area a characteristic 
appearance. The hairs are provided with very small sebaceous glands ; the coiled 
tubular glands are much larger, and open near hairs disposed singly. In Man 
the tubular glands are less conspicuous. ; 
