URINOGENITAL ORGANS 



443 



mesoderm below, the ccelome primarily extending upwards through 

 both nephrotome and myotome (cf. Fig. 335). 



Pronephros. 



As a rule, the rudiments of the pronephros can be recognised in 

 the anterior segments of the trunk, and its glandular canals, 

 extending in the transverse plane of the body, must be distin- 

 guished from the primary pronephric (ctrchinephric or segmental) 

 duct (Figs. 335-337), which passes backwards to open into 

 or close behind the cloaca. Each canal communicates on the one 

 hand with the ccelome by means of a ciliated, funnel-shaped 

 aperture or nephrostome, and on the other with the pronephric 

 duct. A projection of the inner wall of the ccelome arises right 

 and left of the mesentery, projecting towards each canal, and into 



Pronephric eapsuhs 



Glomerulus 



Nephrostome 



FIG. 336. DIAGRAM OF THE PRONEPHROS SHOWING THE FORMATION OF THE 



GLOMERULI. (After Felix.) 



A broad strip of the ectoderm and the ventro-lateral portions of the mesodermic 

 segments are removed, and the pronephric duct and tubules are seen above 

 the dorsal side of the ccelomic wall (transversely lined). 



this branches from the aorta extend, each giving rise to a rete 

 mirabile consisting of a coiled tuft of capillaries or c/lomerulus, by 

 means of which water is filtered out from the blood : several 

 glomeruli may unite to form a glomus (Fig. 337). In many 

 Vertebrates, the parietal and visceral layers of the peritoneum in 

 the anterior part of the ccelome unite around the glomeruli 

 and nephrostomes so as to form ccelomic pronephric chambers or 

 capsules more or less completely shut off from the rest of the 

 body-cavity (Fig. 336). 



In Myxinoids the glandular pronephric rudiment extends along 

 almost the whole length of the ccelome, but in other Vertebrates 



