gall bladder 

 liver 



yolk stalk 



liver 



ventral pancreas 



dorsal pancreas 



gall bladder 



bile duct 

 dorsal pancreas 



internal yolk sac- 



bile duct_ 

 yolk stalk - 

 pancreatic duct- 



STAGE 21 



B 



STAGE 24 



D STAGE 25 + 



Figure 9-24. Three stages in the development of the spiral valve of the shark Pristiurus. The stip- 

 pled tissues represent the lining of the gut, and several cross sections of stage 24 show the relation- 

 ships of this lining to the rest of the v/all of the gut and the subintestinal vein. (After Ruckert, 1897) 



creatic acini of higher forms. These patches can only be 

 observed in prepared sections. The anterior pocket of the 

 intestine has been viewed as a forerunner of the pancreatic 

 diverticulum, but it might equally well be considered a 

 pyloric caecum. 



The development of the islet tissue can be observed in the 

 25 or 30-mm lamprey where small tubules extend out into 

 the overlying connective tissue in the area of entrance of 

 the bile duct The tubules do not open into the gut cavity 

 but connect only with the outer surface of the gut wall. 

 Such tubules grow outward from the ventral, left, and dor- 

 sal aspects of the gut, and none is connected with the bile 

 duct itself 



The typhlosole of the gut is generally described as a 

 spiral valve, but it does not spiral in a way comparable to 

 that of higher groups. In the lamprey this fold tends to lie 

 along the left ventrolateral wall of the gut. In the adult the 

 anterior end of the valve is left dorsolateral, so little rotation 

 is evident. The gut, which lies free in the body cavity, ap- 

 pears to be twisted a turn or two in the region behind its mid- 



dle. The valve tends to spiral, following it from behind for- 

 ward, in a counterclockwise direction in the posterior third, 

 and in a clockwise direction in the next third. The typhlo- 

 sole of the lamprey is peculiar in that the mesenteric artery 

 passes posteriorly through it, a situation not observed in 

 higher forms. 



EMBRYOLOGiCAL DEVELOPMENT The lamprey egg undergoes 

 holoblastic equal cleavage (Figure 7-6). The yolk is con- 

 tained within the cells of the gut wall. At first the gut wall 

 is many cells thick, and irregularly disposed spaces coalesce 

 to form the definitive lumen. The blastopore remains as the 

 anus, and there is no proctodeum. The liver appears first as 

 a ventral evagination into the yolk mass from the anterior 

 end of the archenteron or primitive gut. Later it is displaced 

 backward and comes to lie behind the pharynx and in front 

 of the yolk-expanded gut wall. 



The development of the gut shows no rotation in the early 

 stages. The typhlosole fold is filled by a mass of lymphoid or 

 hemopoietic tissue. 



276 



THE VISCERA 



