'A^iiver 



esophagus 



gall bladder 



pyloric valve 



yolk sac {periblast enclosed 



part stippled) 



intestinal portal 



stomach 



spiral valve 



bile duct 



pancreas and duct 



stomach 



spleen 



Figure 9-18. Two stages in the development of the digestive tract of Amio. A, early embryo in 

 dorsal view; B, 22-mm embryo in dorsal view. 



part of the intestine. This gland varies much in size; it is 

 small in the rays, larger in the sharks. This compound tu- 

 bular gland appears to function in control of blood salts. 



The liver is bilobed; the lobes are joined anteriorly, ven- 

 tral to the esophagus. The gall bladder is large and embed- 

 ded in the right lobe near the ventral midline. The pan- 

 creas is compact but irregularly shaped and contains islet 

 tissue. It lies below the anterior end of the intestine and ex- 

 tends from here to the left below the pyloris. The spleen lies 

 dorsal to the posterior end of the body of the stomach and 

 extends around, behind and below, or along the pyloric 

 section of the stomach. The spleen may extend to the right 

 to underlie the pancreas and pieces of the spleen may ex- 

 tend forward to the dorsal mesentery of the esophagus. 



A dorsal mesentery is present for the stomach and the 

 anterior part of the intestine, also for the rectal gland. 

 There is no ventral mesentery. Lymphoid and fatty tissue, 

 other than the large spleen, is not evident. 



EMBRYOLOGicvL DEVELOPMENT The entoderm is divided 

 early into a syncytial periblast on the surface of the yolk 

 and the cellular entoderm of the blastodisc (Figure 7-10). 

 The embryo develops at the posterior margin of the blasto- 



disc but involves somewhat more complex movements than 

 the actinopterygian. 



Concrescence brings the cellular materials of the epiblast 

 toward the midline of the posterior margin of the blasto- 

 disc, and this is accompanied by forward and lateral move- 

 ments in the hypoblast. In the prechordal area the hypo- 

 blast is not already vertically delaminated into entoderm 

 and mesoderm As the embryo is added to posteriorly, this 

 separation gradually appears. The mesoderm arises from 

 the hypoblast along the midline, while the dorsal gut wall is 

 produced by movements of the more lateral parts of the 

 hypoblast medially below the mesodermal portion. In the 

 actinopterygians the cells of the caudal node separate into 

 entoderm and mesoderm as the embryo is laid down along 

 the midline. 



The foregut is formed by the separation and forward 

 growth of the head fold, the hindgut by the formation and 

 separation of the caudal end (Figure 9-23). As these two 

 ends increase in length, the yolk sac constricts and comes to 

 hang well below the developing animal on a slender stalk. 

 At this time the "foetus" is developing in the egg shell or 

 the uterus of the mother. Absorption of the yolk is accom- 

 plished by shortening of the sac and drawing it into the 



272 • THE VISCERA 



