DEVELOPMENT OF THE EMBRYO. 41 



come into existence. The details of this process it is the 

 province of the science of embryology to consider. 



Well-defined tracts of the adult body may be traced back to 

 the three primary blastodermic layers, and the distinctions of 

 origin thus made are commonly emphasized as of importance. 

 Without entering into minutiae, the tissues derived from the 

 primary layers are in general as follows : 



From the epiblast are derived the epithelium of the epider- 

 mis ; the nerve-centres and axis-cylinders, and the enamel of 

 the teeth. 



From the hypoblast is derived the epithelium of the mucous 

 membranes lining the alimentary and respiratory tracts and 

 bladder, and of the glands tributary thereto. 



From the mesoblast are derived the corium of the skin and 

 the tunica propria of mucous membranes ; the connective-tissue 

 structures ; cartilage, bone, the vascular and lymphatic sys- 

 tems, the blood, the muscular tissues, the reproductive organs, 

 and the kidneys and ureters. 



