272 THE FIFTH DAY. [CHAP. 



and the dermis. From the epiblast are formed all such 

 tegumentary organs or parts of organs as are epidermic 

 in nature. 



In addition to these, the epiblast plays an important 

 part in the formation of the organs of special sense. 



According to their mode of formation these organs 

 may be arranged into two divisions. In the first come 

 the cases where the sensory expansion of the organ of 

 special sense is derived from the involuted epiblast of 

 the medullary canal. To this class belongs the retina, 

 including the epithelial pigment of the choroid, which 

 is formed from the original optic vesicle budded out 

 from the fore-brain. 



To the second class belong the epithelial expansions 

 of the membranous labyrinth of the ear and the cavity 

 of the nose, which are formed by involution from the 

 superficial epiblast covering the external surface of the 

 embryo. These accordingly have no primary connection 

 with the brain. We may also fairly suppose that the 

 'taste bulbs' and the nervous cells, which have lately 

 been described as present in the epidermis, are also 

 structures formed from the epiblast. 



In addition to these we have the crystalline lens 

 formed of involuted epiblast, and the cavity of the 

 mouth and anus lined by it. The pituitary body is 

 also epiblastic in origin. These are the most important 

 parts which are derived from the epiblast. 



From the hypoblast are derived the epithelium of 

 the digestive canal, the epithelium of the trachea, 

 bronchial tubes and air cells, the cylindrical epithelium 

 of the ducts of the liver, pancreas and other glands of 

 the alimentary canal, as well as the hepatic cells con- 



