DERIVATIVES OF THE MIDGUT 189 



lates the various regions of the nervous system in the overlying ectoderm. 

 May we not suppose that the primitive heart stimulates the various regions 

 in the overlying endoderm? Only experiment can answer such a question, 

 but the contact relationship between the heart and the endoderm suggests 

 strongly an induction relationship. 



Derivatives of the midgut 



There is some experimental support for this suggestion in the case of 

 the development of the liver. Figure 113 shows the liver arising as two 

 forward-directed outgrowths of the endoderm even before the midgut closes 

 up. These tonguelike diverticulae crawl along the ductus venosus and break 

 up into a profusely branched system encircling this vein. And, as we have 

 seen from a study of circulation in a later embryo, the blood from the vitelline 

 veins finally flows through the spaces in the liver and is collected by the 

 hepatic veins of the ductus venosus. The fact that the endoderm grows along 

 the veins and around the ductus venosus strongly suggests that these vessels 

 stimulate the first formation and later growth of the liver. 



More direct evidence comes from experiments in which parts of the 

 primitive streak embryo are cultured on the chorioallantoic membrane. Here 



.GALL B LADDER 

 POSTERIOR LIVER 

 ANTERIOR LIVER 

 Fig. 113. The formation of the liver and pancreas. The midgut remains open 

 to the yolk, and its ventral wall curves ventrally below the heart. Thus the two 

 liver outgrowths in the figure are at first dorsal and ventral. They are termed 

 anterior and posterior because when the midgut begins to close these liver out- 

 growths assume an anterior and posterior position. The gall bladder appears as a 

 secondary outgrowth of the posterior liver. The pancreas originates as three out- 

 growths of the midgut — one dorsal and two ventral, 



