812 DEVELOPMENT OF THE UEO-GEXITAL OEGANS. 



organ rei:)resenting the clitoris or penis. Into this jirominence the 

 nrogenital groove runs forward, and surrounding the prominence in 

 front, and continued downwards on each side of the urogenital opening, 

 there is a raised ridge of integument, which is the foundation of the 

 future lal)ia majora in the female, and of the two halves of the scrotum 

 in the male. 



The description of the later changes which occur in these parts in 

 the development of fuller sexual differences will be given hereafter. 

 Here it will be sufficient to state their general nature. In embryoes 

 which are assuming the male type, the common eminence becomes 

 gradually longer, more cylindrical and deeply grooved along its lower 

 surface. The lateral ridges of the urogenital opening become united 

 from behind forwards along the middle line, and this union is gradually 

 continued into the ridges of the groove below the penis, so as to 

 enclose a canal which becomes the urethra with its tegumental and 

 spongy vascular coverings, and to form below this the scrotum, in which 

 the raphe is the remains of the median union of the integmnent. 



In female embryoes, on the other hand, the cylindrical eminence 

 remains comparatively small, and the groove along its lower surface 

 ■widens into two folds, forming the laljicc minorcc or nymphae ; while the 

 larger lateral integumental folds, retaining their prominence and remain- 

 ing separate, constitute the labia majora. The groove is not closed, but 

 widened and shortened so as to become the vulva, while more deeply 

 the sinus urogenitalis shortens itself considerably so as to form the 

 limited atrium vagiiuc, into which open the urethra from the urinary 

 bladder and the now united lower portion of Midler's ducts forming the 

 yagina. 



From the previous statement, it appears that both the urinary and the 

 reproductive organs take their origin in symmetrical pairs from the 

 intermediate cell-masses of the mesoblast, which are situated to the out- 

 side at first, and subsequently below, the protovertebral columns. The 

 earliest formed of these organs are the Wolffian bodies, by which the 

 others are all intimately connected together in their development, so as, 

 to form one great system. It further appears that, while the urinary 

 organs are developed in an entirely similar manner in all embryoes, 

 there are in the sexual organs certain departures from the common type 

 b}^ which the peculiarities of the male and female are established. The 

 general plan of development of these organs having been previously 

 described, the history of the process will now be completed by an 

 account of the further changes which they undergo. 



FURTHER HISTORY OF THE3 DEVELOPMENT OF THE UROaENITAL 



ORGANS. 



Tlie Kidneys and their Ducts. — -These organs are developed togetlier from 

 a mass of formative cells situated posteriorly on the dorsal aspect of the Wolffian 

 bodies, their first hollows being formed as diverticula from the Wolffian duct. 



The formative blastema of the kidney, as observed 133- Eathke in the foetal calf, 

 soon contams a series of club-shaped bodies v^'hich have their larger ends free and 

 turned outwards, and their smaller ends or pedicles directed inwards towards the 

 futiire hilus, where they are blended together. As the organ grows these bodies 

 increase in number, and finally, becoming hollow, form the uriniferous tuhes. At 

 fii-st, short, wide, and dilated at their extremities, the tubuli soon become elongated, 

 narrow, and flexuous, occupying the whole mass of the kidney, which then appears 

 to consist of cortical substance only. At a subsequent period, the tubuli nearest 



