left pronephros 



duct of Cuvier 



nephrtc duct 



kidney 



glomerulus 

 Bowmon's capsule 



neck section ____y^ 



2=srE;32jj^__segmental artery 



posterior cardinal (subcordinol) channel 

 segmental vein 



segmental nerve 



USUAL 



B 



dorsal aorta 



Figure 10-30. Kidney of Myxine. A, anterior end of kidney as seen from below, and relationships 

 to blood vessels; B, two types of tubules observed in the adult kidney. (A after Marinelli and Strenger, 

 1956; B after Conel, 1917) 



lost and the coelom extends to what was their medial walls. 

 Segmental funnels form and the cavities of these extend 

 into the pronephric ridge or cord to form the nephric duct. 



Posterior to the 30th somite a different process occurs: 

 there is a transition from one process to the other in the region 

 of the 30th somite. The second process involves retention of 

 the nephrocoels as separate spaces opening into the coelom 

 laterally. The roof of the nephrotome is evaginated upward 

 as a pocket, and the outpocketing of adjacent nephrotomes 

 join dorsally as a continuation of the nephric duct. These 

 segmental outpocketings can be viewed as enlarged funnels. 

 Occasional less developed segments temporarily break the 

 continuum of the nephric duct from front to rear. Nephric 

 elements arising in this way extend back to about the 70th 

 somite. The most posterior elements are rudimentary and 

 abort early; they do, however, contribute to the nephric 

 duct which thus reaches the cloaca. 



In the opisthonephric region, behind the 30th somite, the 

 nephrocoel of each segment is cut off from the coelom by 

 closure of its lateral wall. This process extends gradually but 

 irregularly posteriorly. With the nephrocoel isolated from 

 the coelom, a glomerulus begins to form medial to the fun- 



nel. The entire nephrotome, except the part involved in the 

 nephric duct, is now reorganized into the adult tubule. The 

 nephrocoel forms the Bowman's capsule. The most posterior 

 tubules degenerate early. Posteriorly there is a continued 

 degeneration of units in an anterior direction, until the de- 

 finitive kidney is achieved, the last opisthonephric unit lying 

 in about the 70th somite. 



Anteriorly the pronephros has been pushed posteriorly by 

 the development of the branchial pouches. In this process it 

 has also become compressed. What had extended through 

 segments 12 to 30 now lies in segments 31 to 33. How many 

 of the original elements are retained is not known, perhaps 

 five to seven units. In this shortening process the nephric 

 duct breaks down and in its place a fibrous mass of tissue 

 develops; this is the central mass which now lies in the 

 blood of the pronephric sinus. A pair or two of glomeruli 

 appear and at first bulge into the body cavity below the 

 pronephric mass. Later each of these is enclosed in a pro- 

 nephric chamber with a peritoneal opening into the coelom. 

 The tubules of the shortened pronephros now begin to 

 multiply by a budding process until the adult condition of 

 hundreds of small funnels and tubules is attained. 



THE EXCRETORY SYSTEM • 317 



