I. URINOGENITAL OROANS. 



a. GENERAL PART. 



The first traces of the urinary and generative organs of Verte- 

 brates arise on the dorsal side of the coelome, right and left of the 

 aorta, and are more or less closely connected with one another, 

 both morphologically and physiologically. 



The part of the urinogenital system first to arise is the paired 

 pronephros and its duct, the pronephric duct. This is the 

 most ancient and primitive excretory organ of Vertebrates ; it is 

 usually restricted to a few of the anterior body segments, close 

 behind the head, whence it is often known as the " head-kidney." 

 It originates primarily as a series of segmentally arranged 

 invaginations of the somatic mesoblast in the region of the ventral 

 section of the mesoblastic somites, these invaginations giving rise 

 to excretory tubules or nephridia (Figs. 273 and 274) ; secondarily, 

 however, in consequence of alterations in the relative rate of 

 growth of the parts, the tubules come to arise in connection with 

 the unsegmented body-cavity. Each tubule opens into the coelome 

 by a ciliated funnel or nephrostome, and comes into relation with a 

 segmental blood-vessel which primarily connects the aorta with the 

 subintestinal vein. These vessels become coiled to form a rete 

 mirabile known as the glomus (Fig. 274). Primarily, as in Chseto- 

 pods, the tubules must have opened at the other end on to the 

 surface independently, through the ectoderm (Fig. 277, A, and comp. 

 Amphioxus, p. 348 and Figs. 219 and 277, a), but this condition is 

 no longer observable in the Craniate, in which they all communi- 

 cate with a longitudinal pronephric duct. The number of nephro- 

 stomes is in most cases not more than two or three. 



The pronephric duct is apparently a later acquisition than the 

 pronephros itself. It first appears in the somatic mesoblast,^ 

 arising by the fusion of the peripheral ends of the pronephric 

 tubules to form a longitudinal collecting tube (Figs. 274, 277, b), 

 which extends backwards to open into the cloaca, thus establishing 

 a communication between the coelome and the exterior. 



^ In Elasmobranchs its origin can be traced to the epiblast. 



