R DEVELOPMENT OF THE UBO-GENITAL OEGANS. 1331 



i 



segments from the 



m those of the later appearing mesonephros. The tubules of the pronephros arise in all the 

 fifth cervical to the third lumbar. 



As the permanent kidney, or metanephros, is developed the mesonephros atrophies ; 

 a portion of it, however, is retained in the male, and forms the excretory apparatus of 

 the testis. The Wolffian duct becomes the canal of the epididymis and the ductus 

 deferens of the adult (see p. 1334). 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 they are only represented in the adult by the vestigial structures present in 

 the broad ligament of the uterus (see pp. 1315 and 1316). 



Wolffian duct 



Kidney bud 



Notochord 



/ Neuial tube 



THE URETER AND PERMANENT KIDNEY. 



The ureter arises as a tubular diverticulum from the W T olffian duct close to the point 

 where the latter joins the cloaca (Figs. 1039 and 1042). This diverticulum is seen first 

 during the fourth week, and grows from behind forwards, dorsal to the body cavity. Even 

 in its very early condition the portion of Rectum 



the outgrowth which lies nearest to the 

 Wolffian duct, and from which the adult 

 ureter is developed, is more slender than 

 the distal part, which becomes branched, 



and grows out to form the pelvis and r 



calyces of the ureter. From the calyces Bladder- "'^P^K 

 numerous collecting tubules grow out into 

 the developing kidney and acquire con- 

 nexions with the glandular or uriniferous 

 tubules of the kidney. The uriniferous 

 tubules of the kidney arise independently 

 of the ureter in a backward prolongation 

 of the tissue which, further forward, gives 

 origin to the tubules of the mesonephros. 

 The tissue in which the permanent kidney 

 tubules arise lies behind the third lumbar 

 segment. The blind distal end of each 

 tubule soon dilates to form a capsule 

 which, becoming invaginated on itself, 



Post anal gut 



the Wolffian duct. (Drawn from 

 stmcted by Prof. F. Keibel. ) 



model con - 



FIG. 1042. TAIL END OF A HUMAN EMBRYO ABOUT 

 25 DAYS OLD. 



encloses a tuft of capillary blood-vessels. The cloaca is dividing into rectal and urino- genital 

 The renal corpuscles arising in this manner subdivisions. The ureter is arising as a bud from 



are found in the human kidney as 

 early as the eighth week. 



As regards their origin in the embryo we distinguish between the collecting tubules 

 and their branches, and the uriniferous secretory tubules of the kidney. The former 

 arise from the calyces of the ureter, and hence are derived from the Wolffian duct ; the 

 latter are formed in mesoderm, known as the metanephric cellmass, which is continuous 

 at its anterior end with the tissue from which the mesonephros is derived. The short 

 ju notional tubules of the adult lie in the region where these developmen tally distinct 

 portions of the kidney unite. 



As the ureter increases in length, it becomes separated from the Wolffian duct, and 

 acquires a distinct opening into the anterior part of the cloaca nearer the head of the 

 embryo than that of the Wolffian duct. This part of the cloaca receiving the ureters 

 becomes the bladder. The kidney is at first a distinctly lobulated body, and shows 

 at birth, and sometimes even in the adult, distinct traces of its original subdivision 

 into lobule. 



The metanephric cell mass, in which the uriniferous tubules arise, lies at first on the medial 

 side of the bud-like outgrowth, which represents the ureter ; at a later time it comes to lie 

 dorsally. As the ureter grows towards the head end of the embryo the cell mass which gives 

 rise to the uriniferous tubules follows it ; hence the metanephric tissue ceases to lie to the caudal 

 end of the mesonephros. As the ureter divides to form the calyces, the metanephric cell mass 

 becomes broken up into cap-like portions, one for each branch of the ureter, and later one for 

 each of the collecting tubes which grow out from the calyces. 



kThe formation of uriniferous tubules within the nephrogenic cell mass is continued until 

 w days after birth. 

 85 a 



