As seen in longitudinal section, the kidney lies below the 

 large posterior cardinal vein into which its venous sinuses 

 open. In the medial wall of the kidney fold is a longitudinal 

 glomerular band served by nearly segmental arteries. The 

 capsules associated with the glomerular band are all coa- 

 lesced into a common cavity from which numerous nephric 

 funnels e.\tend in every direction (except directly medially). 

 The nephric tubules are complexly looped and join the 

 nephric duct in the ventral margin of the kidney fold. 



Further details of structure can be seen in a cross-section 

 cut from a preserved specimen (Figure 10-26). Below the 

 notochord is the dorsal aorta and to either side of this the 

 posterior cardinal veins. Separating these vessels from the 

 kidney is a thick layer of connective tissue which appears to 

 be perforated only by the occasional renal arteries serving 

 the glomerular band. Below this connective tissue plate is 

 a thick-walled suprarenal sinus which extends to either side 

 of the midline and is crossed dorsoventrally by numerous 

 trabeculae of connective tissue. Where renal arteries pass 

 down, they are enclosed in broad connections between roof 

 and floor. The trabeculae and the larger connectives enclos- 

 ing the arteries form bilateral anteroposterior bands. These 

 bands support the mesentery of the median gonad. 



The kidney tissue lies below the suprarenal sinus to either 

 side. It is divided into a dorsal, darker part and a larger, 

 ventral, lighter colored region. The dorsal part is lymphoid 

 in nature, producing blood cells, while the white part is the 

 tubular excretory portion. The excretory tissue extends 

 through about half the length of the body cavity, from the 

 40th or 45th myotome to the 75th or 80th myotome. The 

 lymphatic tissues extend the entire length of the body cavity; 

 anteriorly the lymphatic bands end to either side of the sinus 

 venosus, about somite 20, in the region of the late larval pro- 



nephros — see below. The suprarenal sinus extends the en- 

 tire length of the kidney. 



Circulation in this kidney involves direct arterial supply 

 to the glomerular band from which the blood percolates 

 among the tubules into the reticulum of the lymphoid part. 

 From here it passes through slit-like openings in the floor of 

 the suprarenal sinus. From the suprarenal sinuses it passes 

 through similar slits in the roof These slits open into a loose 

 tissue lying along the ventromedial walls of the posterior 

 cardinals. Perhaps this is a phagocytic tissue along with the 

 interrenal tissue. The blood leaves this tissue by round 

 openings, several to a segment, and enters the posterior 

 cardinals. 



EMBRYOLOGICAL DEVELOPMENT The pronephros arises in 

 metotic somites 4 to 19, approximately (Figure 10-28). The 

 nephrotomes develop interior hollows, the nephrocoels, 

 which later, in segments 7 to 1 1 or 12, open laterally into 

 the segmental coelomic spaces (Figure 10-29). The outer 

 layer of the nephrotomes of these segments, and those pos- 

 terior to them, evaginate upward to form a pronephric 

 crest. These crests join from segment to segment. In the most 

 anterior segments, 4 to 6, regression occurs early, but in 

 somites 7 to 11 funnels are formed which extend upward 

 into the pronephric ridge. Behind these the pronephric ridge 

 separates as a cord of cells which later hollows out as 

 the nephric duct. The nephric duct is continued posteriorly 

 by segmental contribution from the nephrotome or inter- 

 mediate plate mesoderm, and in embryos of 34 or 35 somites 

 it reaches the cloaca. 



The tubules of the definitive pronephros vary in number 

 from four to six. They lengthen rapidly and become com- 

 plexly looped (Figure 10-29 B,C). The whole pronephric 



otic capsule 



10th somite 



nephrocoels and funnels 



neuropore 

 nosohypophyseal invagination 



nephric duct blastemas 



Figure 10-28. Lamprey larvo of 26 to 28 somites. (After Domas, 1944) 



314 -THE UROGENITAL SYSTEM 



