IV 



PRONEPHROS 



223 



of the nephrostome the cells become pigmented and carry powerful 

 flagella. At the lip of the nephrostome the lining epithelium of the 

 tubule is continued into the flattened epithelium lining the splanch- 

 nocoele which is richly ciliated in its immediate neighbourhood. 



The archinephric duct is continued back from the pronephros 

 along the coelomic roof to open at its hinder end into the cloaca. 



The pronephros has very characteristic relations to the blood- 

 vascular system. The tubules serve to transmit to the exterior the 

 fluid secreted by the coelomic epithelium and a patch of this epi- 

 thelium, lying on the roof of the splanchnocoele at its mesial side 

 and facing the nephrostomes, has its secretory activity much exag- 

 gerated. This specially secretory epithelium has its area increased 

 by bulging into the 

 splanchnocoele, the 

 bulging portion enclos- 

 ing a vascular skein 

 connected with the 

 aortic root. This bulg- 

 ing structure is known 

 as the glomerulus 

 (Figs. 120 and 121, gl). 

 The anterior convoluted 

 part of the archinephric 

 duct and the tubules 

 opening into it have 

 other relations to the 

 vascular system, for their 

 surface is bathed by the 

 blood of the posterior 

 cardinal sinus which 

 forms a system of ir- 

 regular spaces between 

 them. This double 

 relation to the blood- 

 system is doubtless correlated with the double function of the organ. 

 It serves in the first place to get rid of watery fluid secreted by 

 the coelomic epithelium, and with this function the glomerulus 

 with its aortic blood supply is concerned; secondly it has to 

 extract poisonous waste products from the circulating blood, and 

 this is done by the wall of the tubule acting on the venous blood 

 which bathes its surface. 



Development of the Pronephros in Hypogeophis. — Hypo- 

 geophis, a member of the Gymnophiona, will be taken as an example 

 of the mode of development of the pronephros for the following 

 reasons : 



(1) In this as in other Amphibians the pronephros still becomes 

 an actively functional organ. Consequently the probabilities are in 

 favour of its developmental processes having departed less from the 



Fig. 121. — Transverse section through a 12 mm. Tadpole 

 at the level of the pronephros. (After Marshall, 1893. ) 



gl, glomerulus ; int, intestine ; I, lung ; It, liver ; M, medulla 

 oblongata ; N, notochord ; ns, nephrostome ; oes, oesophagus ; 

 op, operculum ; pn, pronephros. 



