122 THE MULLERIAN DUCT. 



consist throughout life of two separate portions, one median and single, the other, derived 

 from the sympathetic ganglia, paired ; in birds, reptiles, and mammals these distinct portions 

 are combined into the two paired organs (Balfour). But in these also, as has been shown by 

 Mitsukuri for mammals, the medullary or nervoua part is at first distinct and outside the 

 cortical, into which it gradually insinuates itself, retaining, however, its connection with the 

 neighbouring sympathetic ganglia. 



The permanent kidneys arise (1) as protrusions from the posterior end of the 

 Wolffian ducts (see fig. 125, C and D, N), which grow forwards towards the lower part 

 of the Wolffian bodies, and form the ureters and the collecting tubules of the kidney ; 

 (2) from a portion of the intermediate cell-mass situated posterior to the Wolffian 

 body, and within which convoluted tubes and Malpighian corpuscles, and eventually 

 the remaining parts of the uriniferous tubules become developed. But before these 

 changes occur in this intermediate cell-mass, it shifts its position relatively to the 

 Wolffian body, eventually coming to lie above and behind that organ. The con- 

 voluted tubes, with their Malpighian corpuscles, appear to be developed independently 

 of the ureter and collecting tubes, as in the case of the Wolffian tubules and the 

 Wolffian duct, a communication between them being only subsequently established. 



The glomeruli are apparent in the eighth week in the human foetus. In the 

 third month the papillae are formed, and in the fourth month the loops of Henle are 

 seen. The tubes are wider in the foetus than in the adult ; the expansion of the 

 kidney as growth advances must therefore be due mainly to an increase in length of 

 the tubules, since new tubules and glomeruli do not appear to be formed. The 

 human kidney is at first lobulated, the lobules corresponding in number to the 

 Malpighian pyramids, but by the end of the first year after birth, the kidneys have 

 usually nearly lost their lobulated appearance. 



The urinary bladder is formed by a spindle-shaped dilatation of the stalk of 

 the allantois (second month). The upper pole of the spindle extends as the urachus 

 into the umbilical cord ; it not unfreqnently remains hollow for some length within 

 the cord (Luschka). The lower pole of the spindle which passes towards the cloaca 

 becomes the first part of the urethra of the male, and the whole of the urethra 

 of the female. The rest of the male urethra is formed and enclosed by the folds of 

 integument which produce the penis (see p. 128). The ureters, which are originally 

 prolonged from and open into the Wolffian ducts, subsequently become shifted in 

 position, so as eventually to open into the enlargement of the allantoic stalk, from 

 which the bladder is formed. 



The Miillerian duct. In lower vertebrates, as was shown by Balfour for 

 elasmobranchs, this duct takes origin by the splitting off of the ventral part of a 

 longitudinal segmental or Wolffian duct, the dorsal part remaining as the Wolffian 

 duct proper, and receiving the segmental and uriniferous tubes, while the ventral part 

 retains the funnel-shaped orifice, by which the segmental duct communicated ante- 

 riorly with the body cavity, and comes to open posteriorly into the cloaca by an 

 orifice distinct from that of the Wolffian duct (fig. 138 and fig. 147). In amniotic 

 vertebrates, the process of formation of a Miillerian duct is somewhat different. 

 It arises on the outer side of the already fairly well developed Wolffian body, and 

 some little distance from the anterior end of that organ, as a thickening of the 

 peritoneal epithelium (fig. 143, a'), which thickening becomes invaginated towards 

 the adjacent Wolffian duct, in the form of three successive funnel- shaped depressions 

 (fig. 148), somewhat similar to those which are connected with the previously formed 

 Wolffian segmental tubes. The invaginations are connected together by a con- 

 tinuous epithelial ridge, forming a cord which becomes disconnected from the 

 peritoneal cavity except at the anterior invagination, and subsequently acquires 

 a lumen. The short tube which is thus formed, soon begins to grow backward 

 as a solid rod of cells, which comes in close contact as it proceeds with the Wolffian 



