DEVELOPMENT OF THE URINARY ORGANS. 947 



is at first situated behind that of the Wolffian duct, from which it is con- 

 stricted off. At first both open together into the Cloaca, but by the sixth 

 day the two ducts have independent openings. From the upper end of the 

 ureter diverticula are given off (c), which seem to sprout from the outer 

 edges of the Wolffian bodies, but which are really independent formations, 

 springing from a mass of blastema behind them ; and as they gradually in- 

 crease in size and advance in development, the Wolffian bodies retrograde; 

 so that at the end of fetal life, the only vestige of them is to be found as a 

 shrunk rudiment, situated (in the male) near the testes, to which their ex- 

 cretory ducts serve as the outlets, becoming the " vasa defereutia." About 

 the end of the fourth day a furrow is formed on each side by an involution 

 of the germinal epithelium below the projection formed by the Wolffian 

 body. The sides of the furrow arch over and unite to form Midler's duct, 

 which at first unites with the Wolffiau duct near the entrance of the latter 

 into the cloaca, but subsequently separately and above it. The duct of 

 Miiller on the right side (in the Fowl, the left with the corresponding ovary 

 disappearing), is never closed in in front, and remains in the female as the 

 oviduct. In the male it is almost entirely obliterated on both sides. The 

 history of the development of the Urinary organs in the human embryo, 

 seems to correspond closely with the foregoing. The Wolffian bodies begin 

 to appear towards the end of the first month ; and it is in the course of the 

 seventh week, that the true Kidneys first present themselves. When at 

 their greatest development, the Corpora Wolffiana are the most vascular 

 parts of the body next to the liver ; four or five branches from the aorta are 

 distributed to each, and two veins are returned from each to the vena cava. 

 The upper arteries and their corresponding veins are afterwards converted 

 into the Renal or emulgent vessels ; and the lower into the Spermatic ves- 

 sels. From the beginning of the third month, a diminution takes place in 

 the size of the Wolffian bodies, pari passu with the increase of the Kidneys ; 

 and at the time of birth scarcely any traces of the former can be found. At 

 the end of the third month, the Kidneys consist of seven or eight lobes, the 

 future pyramids ; their excretory ducts still terminate in the canal, the sinus 

 urogenitulis, which receives those of the Wolffian bodies (subsequently to 

 become the vasa deferentia), and of the Fallopian tubes j 1 and this opens, 

 with the rectum, into a sort of Cloaca, analogous to that which is permanent 

 in the oviparous Vertebrata. The Kidneys are at this time covered by the 

 Suprarenal capsules, which equal them in size ; about the sixth month, 

 however, these have decreased, whilst the kidneys have increased, so that 

 their proportional weight is as 1 to 2?. At birth, the weight of the Kidneys 

 is about three times that of the Suprarenal capsules, and they bear to the 

 whole body the proportion of 1 to 80 ; in the adult, however, they are no 

 more than 1 to 240. The tabulated appearance of the kidney gradually 

 disappears ; partly in consequence of the condensation of the areolar tissue 

 which connects its different portions, and partly through the development of 

 additional tubuli in the interstices. The Urinary Bladder is formed quite 



i Although it has been usually considered that the Vasa Dcferentia of the male 

 and the Fallopian tubes of the female are homologous organs, yet tin's does not seem 

 really to be the ease; for the former are derived from the excretory ducts of the 

 AVolffian bodies, whilst the latter are independent formations, which are found to 

 coexist with seminal ducts at an early period of development, alike in male and in 

 female embryos. (See Kobelt, Der Nebeneierstock des Weibes, Heidelberg, 1847.) 

 The ducts of the Wolffian bodies, although subsequently disappearing in the females 

 of most Mammals, remain permanent as " Gaertner's canals " in the female .Rumi- 

 nants and Pig. 



