KIDNKV 



185 



tubules, the neogenic zone does not form merely a layer on the surface, but extends between the 

 groups down to the pelvis as a series of septa, the ' primary columns of Berlin' (Haugh). The 

 primary collecting ducts are at first relatively far apart, but as the division progresses they 

 come to be set at sharp angles to one another, until at last they form the straight tubules of the 

 pyramids and medullary rays. 



The urinary bladder is developed from the ventral portion of the cloaca 

 entodermica, which, as we have already seen, becomes divided by a septum into 

 rectum and urogenital sinus. The sinus and the allantois now form a tubular passage, 

 on which a dilatation appears implicating the section derived from the cloaca, 

 and perhaps also part of the allantois. This becomes the bladder, and the allantois 

 is obliterated to form the urachus. When the division of the cloaca is effected 



junctionnl tubule 



ggj- 2nd convoluted 



tubule 



desc. limb of Henle'n- 

 tubule 



afferent ) 

 efferent f 



Bowman's 

 capsule 



asc. limb of Herile's tubule 



Fiw. 2 ::}. -DIAGRAMMATIC SECTION OF A DEVELOPING KIDNEY-TUBULE AND OLOMERULUS. 



(T. H. Bryce.) 



the Wolffian ducts come to open into the urogenital sinus. The openings are at 

 first common to the ducts and the ureters, but soon they are caused to open 

 separately by the lower ends of the Wolffian ducts being taken into the wall of 

 the sinus. The portion of the sinus intervening between the pairs of openings 

 then elongates, and the relations are so altered that the ureters open into the 

 bladder dilatation, and the Wolffian ducts into the sinus proper. This ultimately 

 forms the prostatic and membranous portions of the urethra in the male, and the 

 whole urethra and the vestibulum vaginae in the female. The Wolffian ducts at 

 first open into the urogenital sinus close together, separated by an eminence in 

 which the fused Miillerian ducts end (fig. 248, p. 196). In the male the eminence 

 persists as the crista uretkralis, which extends also along that section of the sinus 

 which becomes elongated as the ureters and Wolffian ducts draw apart. When, 



