1142 UROGENITAL SYSTEM. 
of transversely-arranged canals or tubules, each of which opens at its inner end into the 
Wolftian duct, while its outer extremity ends blindly. These transverse tubules, like the 
canal into which they open, are at first solid cellular structures, and only later acquire a 
distinct lumen. Incre asing rapidly in size and number, the tubules become twisted and 
tortuous, and the free outer end of each dilates to form 
a capsule invaginated upon itself and containing a 
bunch of capillary blood-vessels similar to the elo- 
meruli of the adult kidney. The number of transverse 
tubules does not correspond to the number of somites, 
but several tubules are present for each somite in which 
a portion of the Wolffian body occurs. The Wolffian 
body is developed throughout the greater part of the 
leneth of the embryo, and reaches from the fifth 
somite in front to the fifth lumbar somite behind. 
The anterior part, which possibly represents the “ pro- 
nephros ” of lower vertebrates, very rapidly atrophies. 
When at its greatest development (about the eighth 
week) the mesonephros forms a relatively large glandular 
mass projecting into the dorsal part of the body cavity, 
and extending from the region of the liver to the 
caudal end of the body cavity. 
In the lower vertebrates the tubules of the Wolffian 
body open into the body cavity by wide ciliated mouths, 
and in these animals the tubules are developed as out- 
growths of the peritoneal membrane. In anamniate 
vertebrates the mesonephros persists as the secretory 
organ of the adult. 
As the permanent kidney is developed the meso- 
nephros atrophies ; a portion of it, however, is retained 
in the male, and forms the excretory apparatus of 
the testis. The Wolfhan duct becomes the canal of 
the epididymis and the vas deferens of the adult (see 
p- 1145). In the female, when the permanent kidney 
is formed, the mesonephros and its duct undergo 
atrophy to a greater extent than in the male, and are 
only represented in the adult by rudimentary struc- 
tures (see p. 1145). 
Ureter and Permanent Kidney.—The kidney 
and ureter arise as a tubular diverticulum from 
eer the Wolffian duct close to the point where the latter 
joins the cloaca. This diverticulum is seen first 
te * during the fourth week, and grows from behind 
SSDER OS WHCH THE/ URET OS SE. forwards, dorsal itoy the body cavity. Even in its 
Vas DEFERENS, AND THE BLADDER 2 ee : p 
ARISE IN THE EMBRYO. very early condition the portion of the outgrowth 
The structures developed from the cloaca Which lies nearest to the Wolfhan duct, and from which 
are indicated in blue, those from the the adult ureter is developed, is more slender than the 
Wolffian duct in red, and the ectoderm distal part, which becomes branched, and grows out 
in black: to form the pelvis and calyces of the ureter. The 
The manner in which the rectum and yyiniferous tubules of the kidney arise as branching 
bladder become separated and acquire + uJar outgrowths from the calyces into the surround: 
openings into the ectodermal cloacal 
ee igehonm in itand tl (A, BH. Ang mesoblast. The blind distal end of each tubule 
Fic. 781.—DIAGRAM TO ILLUSTRATE THE 
Young and A. Robinson). soon dilates to form a capsule which, becoming in- 
A. Allantois. R. Rectum. vaginated on itself, encloses a tuft of capillary blood- 
é Ph -. Urogenital canal. aa a ere epic te ic 2 
Beg nce Ur Drogenital cantl- vessels, The Malpighian corpuscles arising in this 
OM. Ectoderm of VD. Vas deferens. manner are found in the human kidney as early as 
cloacal fossa. VS. Seminal vesicle. 3m 7 Y 
K. Pelvis of kidney. WD. Wolffian duct. the eighth week. 
According to some embryologists it is only the 
collecting tubules that are formed in this manner, and the other tubules of the kidney 
have a mesoblastic origin independent of the outgrowth which gives origin to the ureter 
and its pelvis. These “authorities state that the kidney tubules arise in the mesoblast, 
and only later acquire a connexion with the outgrowths from the ureter. Recent 
observation, however, does not support these statements. 
The kidney is at first a distinctly lobulated body, and shows at birth, and sometimes 
