URINOGENITAL ORGANS 83 



The dorsal and ventral roots of the spinal nerves of the trunk 

 always combine to form mixed branches. The sympathetic 

 nervous system becomes well differentiated, with segmental and 

 visceral ganglia. 



Other, and perhaps more important, differences are found in 

 the structure of the urinogenital organs (p. 27). As already 

 mentioned, we find a succession of excretory organs in the 

 ontogeny of the Craniata, which is more thoroughly carried out 

 in the Gnathostomes than in the Cyclostomes. Something must 

 here be said concerning the development of these organs in the 

 Craniata generally (see also p. 365). 



A very large number of embryologists, among whom one may 

 mention Balfour [26, 29], Sedgwick [395-6], Ruckert [376-7], 

 Rabl [337], Semon [397], Brauer [45], Kerens [258], van Wijhe 

 [496], Field [137], Wheeler [486], Price [335], and Felix [135-6], 

 have studied this subject in various groups. Felix has recently 

 published an excellent summary of our present knowledge of it 

 [136]. 



A pronephros is found in the embryo of all Craniates, although 

 it generally appears to be incompletely developed, or degenerate. 

 In Elasmobranchs and Amniota it is vestigial and probably never 

 functional. 



In the early stages of development the segmented mesoblastic 

 somites separate off gradually from the unsegmented lateral 

 mesoblast (p. 3 ), a slender stalk alone for a time connecting 

 the two together. It is from this connecting stalk, gener- 

 ally containing a lumen whereby the myocoel communicates 

 with the splanchnocoel, that the pronephric tubules arise, either 

 directly or indirectly. This rudiment of the pronephric tubule, 

 the nephrotome, may be nipped off as a solid block, which subse- 

 quently acquires a lumen and joins on to the lateral plate, or it 

 may from the first be fused with the lateral plate (Teleost). In 

 the typical fully developed organ each tubule resembles a 

 mesonephric tubule, and consists of a segmental ciliated funnel 

 opening into the coelom, the coelomostome (outer funnel, or 

 primary nephrostome of the communicating canal). This leads by 

 a narrow canal (Erganzungskanal) to the renal chamber or capsule 

 (Bowman's capsule of the Malpighian body, the ' urocoele '). Into 

 this small chamber opens a funnel (inner funnel, or ' urostome ') 

 leading into the main renal canal. The renal capsule arid its 

 canal arise as an outgrowth of the tubule. All the pronephric 

 tubules join a longitudinal duct, which passes backwards to open 

 into the cloaca. This is called the pronephric duct, or segmental 

 duct. Into the renal chamber projects a blood-vessel, supplied 

 from the aorta, known as the glomerulus. 



