198 THE FIFTH DAY. [CHAP. 



sense is derived from the involuted epiblast of the medullary 

 canal. To this class belongs the Retina, including the epi- 

 thelial 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 lined by it. 

 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 constituting the parenchyma of 

 the liver, developed as we have seen from the solid hypoblast 

 cylinders given off around the primary hepatic diverticula. 



Homologous probably with the hepatic cells, and equally 

 of hypoblastic origin, are the spheroidal 'secreting cells' of 

 pancreas and other glands. The epithelium of the salivary 

 glands, though these so exactly resemble the pancreas, 

 is of epiblastic origin, inasmuch as the cavity of the mouth 

 (Chap. VI. § 8) is entirely lined by epiblast. 



The hypoblast also lines the allantois. 



Fromx the mesoblast are formed all the remaining parts 

 of the body. The muscles, the bones, the connective tissue 

 and the vessels, both arteries, veins, capillaries and lymphatics 

 with their appropriate epithelium, are entirely formed from 

 the mesoblast. 



All the nerves of the body, both the cranial nerves (the 

 so-called optic and olfactory nerves alone excepted), the 

 spinal nerves, 'and the sympathetic system, are also formed 

 from the mesoblast. The nerve-cells of the sympathetic 

 ganglia as well as those of the ganglia on the posterior roots 

 of the spinal nerves are of mesoblastic origin, and thus appa?- 



