to pass around the yolk mass as the vitelline veins. The 

 vitellines may become separated from the subintestinal by a 

 network of fine channels {Aapenser), or they may form a 

 ring sinus (salmon) for draining such a vascular net, or they 

 may fuse to form a midline sinus, often identified as part of 

 the sinus venosus. The ducts of Cuvier may extend out around 

 the yolk sac to enter the sinus venosus or they too may form a 

 network of fine channels. The reticulum of vessels on the 

 yolk sac serves for the respiratory as well as for the nutritive 

 needs. 



Anna in its development seems typical in most respects of 

 the ray-finned fishes. The caudal vein arises in a ple.xus of 

 three or four small vessels ventral to and irregularly con- 

 nected with the caudal artery. The posterior cardinals con- 

 nect with this caudal channel, and each is formed of a 

 reticulum of vessels around the nephric duct. Of these chan- 

 nels the medial one, the subcardinal, is the larger. The sub- 

 cardinals of either side are interconnected at intervals. Ante- 

 riorly each postcardinal forms a pronephric sinus emptying 

 into the duct of Cuvier. The ducts of Cuvier extend ventro- 

 laterally around the yolk sac, receiving many of the channels 

 of the yolk sac; they enter the sinus venosus ventrally. 



From the caudal vein, connectives pass to either side of 

 the postanal gut. These are interconnected above and below 

 the gut. In the anal region a large channel passes down 

 medial to the nephric duct of either side to form the subin- 

 testinal vein and the plexus of vessels enclosing the gut. 

 This plexus is drained by dorsal mesenteric vessels joining 

 the postcardinals as well as by way of the subintestinal 

 which leads into the yolk-sac reticulum. The capillaries of 

 the liver are continuous with the vessels of the yolk sac to the 

 left of the anterior intestinal portal. 



Later the left postcardinal is reduced and the right be- 

 comes the sole channel of the posterior part of the kidney. 

 The subintestinal connects around the left side of the gut 

 with the posterior cardinal and with dorsal sinuses leading 

 to the yolk sac and liver on the right. The definitive hepatic 

 portal stem follows the same course as in other vertebrates: 

 up on the left side and over the gut into the liver. 



In actinopterygians the yolk sac is usually attached to the 

 gut by a narrow stalk, and the subintestinal channel may be 

 almost entirely associated with the yolk sac. In the salmon 

 the yolk mass lies far forward. The subintestinal passes up 

 to the left of the gut and over the gut to reach the liver. From 

 the liver the hepatic channels extend out to the yolk sac re- 

 ticulum which is drained by the vitelline veins. Earlier 

 stages of the salmon have the subintestinal feeding directly 

 into the yolk-sac circulation. 



Acipenser is the exception in yolk sac-gut relationships. In 

 this the yolk lies in the foregut, the liver diverticulum arises 

 behind the yolk mass (Figure 9-17). In later stages the 

 stomach is much expanded but there is yolk throughout 

 much of the gut. The vascular relationships appear to be 

 similar to Salrno or Amia — there are more or less separate 

 but interconnected yolk-sac and liver circulations. 



Chondr/chrh/es In the shark the same general pattern of 

 vessels is observed as in the other fishes. The posterior cardi- 

 nals are broken into a renal portal vessel and a subcardinal 

 channel which anteriorly leads into the outer postcardinal 

 channel. In place of a ventral abdominal vein there are 

 lateral abdominal veins draining the body wall. Such veins 

 are represented in the amphibia by the musculocutaneous 

 veins. Posteriorly there is a caudal vein, and there are fin 

 veins that enter the interconnected system of renal portal 

 and lateral veins. 



EMBRYOLOGicAL DEVELOPMENT The development of the yolk- 

 sac circulation and the hepatic portal stem in this group 

 illustrate the functional plasticity of this system. The most 

 complete account of the very early stages is that of Torpedo 

 (Figure 11-40). As in other vertebrates, the first indications 

 of the blood system are the "blood islands" formed in the 

 area opaca of the blastodisc at the two-somite stage. These 

 increase in number and anastomose as the embryo begins 

 to rise up from the blastodisc. The blood islands fuse to form 

 two sinuses, an outer terminal sinus around the outer edge 

 of the expanding blastodisc and an inner ring sinus sur- 

 rounding the mesoderm-free area of the blastodisc anterior 

 to the head region of the embryo, the proamnion. 



The first vessels to appear (the exact sequence is doubtful) 

 are the anterior vitelline veins which fuse to form the heart. 

 These veins drain the inner ring sinus. The development of 

 blood vessels in the body begins with the formation of the 

 ventral aorta, a pair of aortic arches, and paired dorsal 

 aortae. In the posterior part of the embryo, which has now 

 separated partially from the yolk sac, appear two caudal 

 veins, closely underlying the aortae and receiving blood 

 from them through connectives. These caudal veins proceed 

 anteriorly and at the yolk stalk curve downward as the pos- 

 terior vitelline veins to enter the terminal sinus. The termi- 

 nal and ring sinuses are connected by the yolk-sac channels, 

 thus completing the circulation. 



At this early period of development, the yolk sac serves 

 as a respiratory organ and in viviparous forms, such as 

 Torpedo, continues to do so until birth. With the yolk-sac 

 circulation completed, changes begin to take place. A small 

 vessel draining the right pronephric sinus into the right an- 

 terior vitelline vein becomes connected with the dorsal 

 aorta. The influx of arterial blood enlarges this vessel and 

 reverses the flow of blood in the right anterior vitelline ves- 

 sel. As a result of this, the anterior connection of the right 

 anterior vitelline vein with the sinus venosus atrophies and 

 is lost. The blood now flows down through the pronephric 

 artery into the right vitelline and through it into the ring 

 sinus and into the heart by way of the left anterior vitelline 

 vein. 



Meanwhile the caudal veins have elongated by the proc- 

 ess of constriction of the yolk stalk and the outgrowth of the 

 tail region and hindgut. In the region of the cloaca, the 

 caudal veins bypass the gut to either side and continue an- 



372 • THE CIRCULATORY SYSTEM 



