THE URINOGEXITAL 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 ccclom 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 

 Miillerian ducts. 



The essential, that is the first, cause of the descensus remains unexplained, 

 and the origin of the ligamentuin 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 syj^athetic 

 nervous_sy_stem. 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. 



