280 EMBRYOLOGY. 



the inner germ-layer, whereas connective tissue, blood-vessels, nerves, 

 and serous covering, although they also belong to these glands as 

 a whole, are of less significance, because the characteristic properties 

 of Uver or pancreas do not depend upon them. In the anatomy and 

 physiology of a muscle the muscular tissue is the more significant 

 part, in the sensory organs the sensory epithelium. 



Guided by such considerations one has a perfect right to designate 

 the intestinal glands as organs of the inner germ-layer, the muscles, 

 the sexual and urinary organs as belonging to the middle germ-layer, 

 and the nervous system together with the sensory organs as products 

 of the outer germ-layer. 



Thus the science of the embryology of organs is divisible into four 

 main sections — into the science of the morphological products of 



(1) the inner germ-layer, (3) the outer germ-layer, 



(2) the middle germ-layer, (4) the intermediate layer. 



