DEVELOPMENT OF THE HUMAN KIDNEY. 417 



Wolffian ducts is identical in both sexes, as is the case in 

 the embryos of Mammals (Fig. 321, G, Fig. 324). In the 

 female Amphibia the Miillerian duct on each side develops 

 into a large ovary (Fig. 322, od), while the Wolffian duct acts 

 permanently as a urinary duct (u). In the male, on the 

 contrary, the Miillerian duct persists only as a rudimentary 

 organ, without functional significance, as Rathke's canal 

 (Fig. 323, c) ; the Wolffian duct serves, in this case also, as a 

 urinary duct, but also as a sperm or seed duct, the seminal 

 tubes (ye) from the testes (i) entering the upper part of the 

 primitive kidneys, and there uniting with the urinary canals. 



In Mammals these conditions, persistent in Amphibia, are 

 rapidly traversed by the embryo in an early period of its 

 development (Fig. 321, C). The primitive kidneys, which 

 in non-amnionate Vertebrates persist throughout life as the 

 urine-secretory organ, are superseded by the secondary 

 kidneys. The actual primitive kidneys disappear almost 

 entirely in the embryo at an early period, leaving but small 

 traces. In the male Mammal the supplementary testis 

 (epididymis) develops from the upper part of the primitive 

 kidney ; in the female the same part gives rise to a useless 

 rudimentary organ, the supplementary ovary (parovarium) 



In the female Mammal the Miillerian ducts undergo very 

 considerable changes. The actual ovaries develop only from 

 its upper part ; the lower part widens out into a spindle- 

 shaped pouch, with a thick, fleshy wall, within which the 

 fertilized egg develops into the embryo. This pouch is the 

 womb (uterus). At first the two uteri are perfectly 

 separate, and open on each side of the urine-bladder (vu) 

 into the cloaca, as is yet permanently the case in the 

 lowest living Mammals, the Beaked Animals (Ornithostoma) ; 



