The liver and gut are not attached to the ventral body 

 wall by a mesentery in any of the actinopterygian fishes. 



Embryologicat development in actinopterygians The devel- 

 opment of the telolecithal egg of the teleost resembles in 

 some respects the development of the chick. In Polyplerus 

 and Aapenser the egg is nearly mesolecithal and the cleavage 

 almost holoblastic (Figures 7-11, 7-12). 



As the head fold is raised off the blastoderm, the foregut 

 is developed. With closure of the blastopore, the tail fold and 

 hindgut develop The hindgut first appears as Kupffer's 

 vesicle. The yolk stalk opens into the midgut. As the embryo 

 increases in size and the yolk diminishes, the yolk sac be- 

 comes enclosed by an embryonic entodermal sac and is 

 drawn into the body. The periblast diminishes with the yolk 

 and disappears with that material as the last of the yolk 

 enters the midgut and the sac is eliminated. 



The liver and pancreatic diverticula develop anterior to 

 the yolk stalk. The liver and gall bladder arise as in the 

 higher forms. Acipenser is the exception to this course of 

 development. A yolk sac is never formed; the yolk is con- 

 tained within the foregut (stomach) anterior to the liver 

 diverticulum (Figure 9-17). 



In Acipenser four pancreatic diverticula have been de- 

 scribed, two dorsal and two ventral. In Amia diere are three 

 diverticula; the dorsal one appears first, then the two ven- 

 tral anlagen. In the 15-mm larva, and the adult, a single 

 duct empties into the gut along with and behind the bile 

 duct. In the salmon only the dorsal diverticulum develops; 

 this lies behind the vertical of the liver diverticulum. There 

 are two ventral diverticula from the bile duct where it enters 

 the gut. The right ventral one becomes the duct of the de- 

 finitive pancreas. All three diverticula are fully developed in 

 some teleosts; the dorsal one gives rise in part to the islet, or 

 islets, of Langerhans. The teleosts in general can be charac- 

 terized as having separate islet masses. These are lacking in 

 the holostean (Amia or Lepisosteus) and chondrostean (Aci- 

 penser) fishes. 



The early development of the hindgut is of special inter- 

 est; it grows in such a way that the typhlosole forms a spiral 



valve. In Polyplerus the spiral valve extends through the en- 

 tire length of the intestine, while in other actinopterygians 

 less than half the length of the intestine is involved. Begin- 

 ning posteriorly the gut lining is rotated through several 

 turns. This rotation is adjusted for largely by shearing move- 

 ment between the cells posterior to the valve. As a result 

 the anterior end of the intestine is rotated in a counterclock- 

 wise direction, as viewed from the rear, about one half turn, 

 so that the bile duct now opens into the gut dorsally. 



In Amia the hindgut, with its typhlosole, lies at first close 

 behind the yolk stalk (Figure 9-18). With the drawing in of 

 the yolk sac, the midgut begins to elongate and to form a 

 loop which extends up on the right side, across dorsally, 

 down on the left side, then medially to the hindgut. The 

 several segments of this loop are elongated in achieving the 

 mature condition. 



In Lepisosteus much the same sequence is observed, but 

 here a pyloric caecum appears as a left ventral diverticulum 

 from the gut (Figure 9-19). Other diverticula develop from 

 this, and each comes to have many tubular extensions until 

 the compact mass of the adult is formed. 



In the actinopterygian the pyloric caeca develop late. In 

 the salmon a few appear, then more and more until the full 

 complement is achieved. Lack of caeca in Amia must be 

 viewed as due to loss. 



Chondrichthyes 



Shark The shark has a wide esophagus and an expanded 

 hook-shaped stomach, generally without a caecum (Figure 

 9-20). Laemargus (and Spmax) are peculiar in having pyloric 

 caeca (Figure 9-21). There is a pyloric valve between stom- 

 ach and gut and sometimes a bursa entiana between stom- 

 ach and intestine (Figure 9-21 C). 



The bile duct opens into the anterior end of the intestine, 

 which has a spiraled (most sharks) or rolled valve {Z\igaena 

 —Figure 9-22). The spiral valve makes from four to fifty or 

 more turns, depending on the species. The valve ends well 

 anterior to the cloaca. A rectal or digitiform gland projects 

 dorsoanteriorly, in the dorsal mesentery, from the terminal 



epithelial plug 



pharyngeal cavity 



mouth 



liver/ 



stomach 



Figure 9-17. Early larva of Acipenser, the sturgeon, showing yoll<-filled stomach and intestine and 

 liver diverticulum. 



THE DIGESTIVE TRACT AND ASSOCIATED STRUCTURES • 271 



