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HANDBOOK OF PHYSIOLOGY 



outer part of these cones involves the omphalo-mesenteric vein, which 

 breaks up in its interior into a plexus of capillaries, ending in venous 

 trunks for the conveyance of the blood to the heart. The inner portion 

 of the cones consists of a number of solid cylindrical masses of cells, 



Fig. 510. Diagram of part of digestive tract of a chick (4th day). The black line represents 

 hypoblast, the outer shading mesoblast ; I g, lung diverticulum with expanded end forming pri- 

 mary lung-vesicle ; St, stomach ; I, two hepatic diverticula, with their terminations united by 

 solid rows of hypoblast cells ; p, diverticulum of the pancreas with the vesicular diverticula 

 coming from it. (Gotte.) 



derived probably from the hypoblast, which become gradually hollowed 

 by the formation of the hepatic ducts, and among which blood-vessels 

 are rapidly developed. The gland cells of the organ are derived from 

 the hypoblast, the connective tissue and vessels without doubt from the 



Fig. 511. Rudiments of the liver on the intestine of a chick at the fifth day of incubation 

 1, heart; 2, intestine; 3, diverticulum of the intestine in which the liver (4) is developed: 5, part 

 of the mucous layer of the germinal membrane. (Miiller.) 



mesoblast. The gall-bladder is developed as a diverticulum from the 

 hepatic duct. The spleen, lymphatic, and thymus glands are developed 

 from the mesoblast: the thyroid partly also from the hypoblast, which 

 grows into it as a diverticulum from the fore-gut. 



