ORIGIN AND DEVELOPMENT 47 



the body. Several layers may combine in 

 the formation of the various organs and 

 tissues. A transverse muscular partition, the 

 diaphragm, divides the body cavity into two, 

 the thorax and the abdomen, 



19. From the ectoderm are derived the 

 epidermic covering, the skin, and structures 

 such as nails and hair ; the epithelium of the 

 glands of the skin, of the mammary gland, of 

 the anterior part of the mouth, of part of 

 the alimentary canal at the anus, of the 

 anterior part of the nasal openings, of a 

 portion of the pituitary body, the enamel of 

 the teeth ; the whole of the nervous system ; 

 the epithelium of the sense organs, and a 

 portion of the suprarenal bodies. It is 

 important to observe that the nervous system 

 and the sensory layers of the organs of special 

 sense are formed from the outer layer of the 

 embryo. 



20. Passing to the inner layer, the endoderm, 

 we find it develops into the epithelium of the 

 alimentary canal and the glands communicat- 

 ing with it ; the epithelium of the two Eusta- 

 chian tubes passing from the throat to the 



