318 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
ureter (fig. 322, 8) 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). A 
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, D), 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 
