318 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



ureter (fig. 322, k) grows forward, parallel to the parent duct, into 

 the tissue posterior and dorsal to the mesonephros. This nephro- 

 genous tissue is apparently serially homologous with that from which 

 the mesonephric tubules have arisen, but all traces of metamerism 

 have disappeared from it. In this nephrogenous tissue the anterior 

 end of the ureter gives off a varying number of branches (fig. 322), 

 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 definitive 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. 323, 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 mul- 

 tiplication of these tubules and an extension of the capillary system 

 of the glomeruli around them, much as in the mesonephros. The 

 differentiation of each tubule into convoluted, collecting and Henle's 

 regions occurs early (fig. 323, 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 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. In the dipnoi there 

 is a diverticulum from the dorsal wall of the cloaca, anterior to the 

 openings of the Wolffian ducts. This is usually called the urinary 

 bladder (fig. 325, Z>), but it may be homologous with the rectal gland 

 of the elasmobranchs. 



The third type, the allantoic bladder, occurs in all tetrapoda. 

 This arises as a ventral diverticulum from the cloaca. In the amphibia 

 the whole of the outgrowth forms the bladder and its walls are sup- 

 plied by the hypogastric arteries. In the amniotes the proximal 

 portion alone is converted into the urinary bladder, while the more 

 distal portion, in the embryo becomes the respiratory organ of the 

 growing young, the allantois. This part extends far beyond the body 

 wall, carrying with it branches of the hypogastric arteries (allantoic 

 arteries), and in the mammals forms a part of the placenta. The 

 allantois becomes reduced in the later stages and at the beginning of 



