308 HISTOLOGY 



between the somites and somatic mesoderm. It lies near the ectoderm, 

 but it is now generally agreed that the ectoderm takes no part in its forma- 

 tion. Finally its growing extremity reaches the ventral portion of the 

 cloaca and fuses with it. Later this ventral part of the cloaca becomes cut 

 off to form the bladder, and the Wolffian ducts then empty into the neck of 

 the bladder. The pronephric tubules meanwhile become detached from 

 the coelomic epithelium, but they remain rudimentary and degenerate 

 without having any glomeruli formed in connection with them. 



The mesonephric tubules develop from the more posterior nephrotomes, 

 after the Wolffian duct has formed. They acquire openings into the 

 Wolffian duct, but do not contribute to its development. In produc- 



ing mesonephric tubules, the 

 nephrotomic tissue becomes de- 

 tached and separates into masses 

 which form vesicles (Fig. 303, B). 

 Each vesicle elongates and be- 

 comes an S-shaped tubule, one 

 end of which fuses with the 

 Wolffian duct and opens into it; 

 the other end remains blind. A 

 knot of capillaries, derived from a 



FIG. 304. RECONSTRUCTION OF A WOLFFIAN TUBULE branch of the aorta, develops in 

 FROM A HUMAN EMBRYO OF 10. 2 MM. (Except the ' '. r ..i o i 



after Koiiman.) the distal concavity of the S and 



c., Inner layer, and c. a., outer layer of the capsule of 11 



the glomerulus; div., diverticulum ; gl., glomerulus; bCCOmCS aglOmerUlUS; 

 W. d., Wolffian duct. ... . . . . 



is formed in connection with every 



Wolffian tubule. The tubules then elongate and become coiled, and 

 together they produce the rounded swellings on either side of the root of 

 the mesentery, which are the Wolffian bodies (Fig. 303, C). The genital 

 glands arise as mesodermal thickenings on the ventro-medial surface of 

 these bodies. 



A single Wolffian tubule is shown in Fig. 304, and the way in which its 

 distal end envelops the glomerulus is clearly indicated. It is said to form 

 the capsule of the glomerulus. By passing through the inner layer of this 

 capsule, fluid from the blood vessels enters the tubule and is conveyed 

 through the Wolffian duct to the bladder. The tubules are generally 

 unbranched, and are lined with simple epithelium. The epithelium is in 

 part glandular, and contributes to the formation of the urine. Finally 

 it may be noted that a nephrotome may divide into several vesicles (some- 

 times perhaps as many as four), and therefore the number of Wolffian 

 tubules is greater than the number of corresponding segments. In man 

 the maximum number is 83 (Felix). The mesonephric tubules also 

 extend forward, so that some segments contain both mesonephric and 

 pronephric tubules. 



