structure bulges into the pericardial cavity as a sac through 

 which the blood of the anterior cardinal vein courses 

 to reach the sinus venosus. The pronephric tubules extend 

 back not only into the duct of Cuvier but even into the pos- 

 terior cardinal channel. The funnels have flaring ciliated 

 margins. The funnel cilia create a current into the tubules. 

 The funnel extends into the pronephric sac a short distance, 

 and joins the tubule which has less intensely staining cells 

 with a ciliated inner surface. On the inner surface of either 

 pronephric sac is a single large glomerulus. 



The pronephros is an active excretory structure in the 

 early larval period. It increases in size (well developed in 

 the 30-mm larva) proportionally as the animal grows, and 

 it does not degenerate until the larva reaches a size of 

 about 100 mm. As the larva increases in size and the bran- 

 chial apparatus expands posteriorly, the pronephros is dis- 

 placed back into segments 18 to 23, approximately. 



The mesonephros is represented at first (10-mm larva) by 

 a thin, irregular blastema of cells arranged around the wall 

 of the lower half of the postcardinal and medial to the 

 nephric duct. The kidney anlage lies ventrolateral to the 

 posterior cardinal and bulges slightly into the body cavity. 

 This ridge increases rapidly in size and forms a band pro- 

 jecting down into the body cavity (Figure 10-29 D). In the 

 15-mm larva about four mesonephric tubules have appeared, 

 about five or six myotomes back of the pronephric area. At 

 their appearance these tubules are quite flexed. The nephric 

 duct lies in about the middle of the kidney band, while the 

 glomeruli are in the ventral margin. In front of and behind 

 these tubules the kidney is lymphoid in nature; dorsally it 

 is not marked off from the posterior cardinal. 



The mesonephros adds new tubules posteriorly with in- 

 creasing size. The most anterior tubules, about 20, regress 

 and disappear, being replaced by lymphoid tissue. As de- 

 velopment proceeds, the number of myotomes between head 

 and cloaca is about doubled. Each segment appears to have 

 a glomerulus and a single initial tubule. There are no peri- 

 toneal funnels. Secondary tubules appear, as do tertiary 

 and perhaps more generations. With multiplication of tu- 

 bules new glomeruli are produced by subdivision of the 

 initial one. In this way a band of four or five glomeruli is 

 produced and is served by a segmental artery (Figure 10-29 

 E). Occasionally segmental arteries extend into the next 

 posterior segment. 



The nephric ducts extend about five myomeres beyond 

 the posterior ends of the kidneys. Here they unite as an ex- 

 cretory sinus into which open, to either side, the posterior 

 pockets of the body cavity. The common duct opens to the 

 exterior through a pore on the tip of the urogenital papilla. 



Myxinid The adult o( Eptalrelus has a pronephros consist- 

 ing of a small mass of tubular tissue just anterior to the duct 

 of Cuvier (the 31st to 33rd somite) and above the heart. The 

 right pronephros is larger than the left and lies somewhat 

 anterior to it. In section, many small funnels are observed 



opening from the coelom into a central mass of cells. The 

 central mass lies inside a pronephric vein. Occasionally 

 channels from funnel to vein are observed. At the posterior 

 end of the pronephros, there is a renal corpuscle opening into 

 the coelom through a ftmnel and into the central mass by a 

 duct. Occasionally two glomeruli occur. 



This head kidney lies well anterior to the opisthonephros 

 (Figure 10-30). The latter is little more than the nephric 

 duct, a thin, slightly looped band beginning at the vertical 

 of the 33rd to 35th myomere and extending for much of the 

 length of the body cavity. The opisthonephros of the male is 

 thicker and somewhat more looped than in the female. Pos- 

 teriorly the nephric ducts join and form a urinary sinus open- 

 ing to the exterior through a papilla (Figure 10-27). The 

 lumps along the length of the opisthonephros represent renal 

 corpuscles and short tubules (Figure 10-30). About four 

 such lumps are crowded at the anterior end, and their 

 arterial and venous connections indicate that they have 

 been displaced backward. The most anterior tubule may 

 lead directly back into the nephric duct, or a vestige of the 

 nephric duct may extend anteriorly beyond the point of en- 

 trance of this tubule. Although the renal corpuscles are ir- 

 regularly spaced, in terms of distribution along the nephric 

 duct or one side as compared with the other, there appears 

 to be one pair of tubules for each muscle segment, in all about 

 35 pairs. 



The tubules are very simple in structure (Figure 10-30 B). 

 There is no peritoneal funnel and cilia are lacking. A thin 

 neck section opens from the capsule into the tubule which is 

 histologically similar to the main nephric duct. In a few 

 specimens of Epiatretus, an elongated neck region opened di- 

 rectly into the nephric duct. The glomerulus is served by a 

 small branch from the dorsal aorta and is drained directly 

 into the postcardinal vein by a short vessel. 



EMBRYOLOGiCAL DEVELOPMENT The development of this 

 type is only partly described. In the earliest embryos, with 

 the same number of somites as the adult (101 to 102), a series 

 of segmental nephrotomes is present in somites 13 to 74. Each 

 is a vesicle, separated from the somite, of dark staining cells 

 enclosing a small nephrocoel. The lateral mesoderm is 

 mesenchymatous at this stage: it is not segmental nor is it 

 divided into somatic and visceral layers. 



New nephrotomes appear posteriorly as those at the an- 

 terior end of the series open laterally into the coelomic 

 spaces. The coelom at first has distinctly segmental cavities 

 medially, but laterally it appears as irregular spaces. All of 

 these spaces unite to form the body cavity, which opens 

 medially into the nephrotomes. Distinct nephrotomes form 

 back to about the 76th somite; behind this are four indistinct 

 nephrotomes in which cavities do not develop, although four 

 less dense areas are indicated. These four incipient neph- 

 rotomes complete the nephric band back to the cloaca. 



The nephrotomic vesicles of segments 12 ( 10 to 13) to 30 

 give rise to pronephric crests dorsally. These crests and the 

 vesicles coalesce; the vesicles as separate entities are now 



316 • THE UROGENITAL SYSTEM 



