THE EXCRETORY ORGANS 



345 



(fig. 369, B), each of which expands at its tip, thus forming a primary 

 renal vesicle, and a little later the place where the branches and the 

 ureter unite expands, the enlargement forming the pelvis of the defini- 

 tive kidney. The cells of the nephrogenous tissue form a number of 

 aggregates around each primary vesicle; each aggregate soon becomes 

 hollow, and develops into an S-shaped tubule (fig. 370, left), one 

 end of which joins the primary renal vesicle, while a glomerulus 

 arises at the other end, but no nephrostomes are formed. Later there 

 is a great multiplication of these tubules and an extension of the 

 capillary system of the glomeruli around them, much as in the 



Fig. 370. — Models of two stages in the development of tubules of kidney (meta- 

 nephros) of man, after Stoerk. b, Bowman's capsule; c, collecting tubule; en, connect- 

 ing tubule; cv, convoluted tubule; A, Henle's loop; i, intercalary tubule; /, lower arch; 

 m, middle piece. 



mesonephros. The differentiation of each tubule into convoluted, 

 collecting and, in birds and mammals, Henle's loop regions occurs 

 early (fig. 370, right). 



Urinary Bladder. — At or near the hinder ends of the excretory 

 ducts there is frequently a reservoir for the urine, the urinary bladder 

 or urocyst. Of these there may be three kinds. In most fishes 

 (fig. 383) the bladder arises by a fusion of the hinder ends of the 

 Wolffian ducts plus a part derived from the hinder end of the digestive 

 tract (cloaca), the Wolffian ducts emptying into it and the whole 

 opening to the exterior, usually dorsal and posterior to the anus 



