COMPARISON OF THE GERMINAL LAYERS. 305 



and the peripheral nervous elements of the body, including both 

 the spinal and the cranial nerves and ganglia, are epiblastic in 



origin. 



The epithelium (ciliated in the young animal) lining the 

 canalis centralis of the spinal cord, together with that lining the 

 ventricles of the brain, is the undifferentiated remnant of the 

 primitive epiblast. 



The epiblast also forms the epidermis ; not however the 

 dermis, which is of mesoblastic origin. The line of junction 

 between the epiblast and the mesoblast coincides with that 

 between the epidermis and the dermis. From the epiblast are 

 formed all such tegumentary organs or parts of organs as are 

 epidermic in nature. 



In addition to the above, 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 organs where 

 the sensory expansion is derived from the involuted epiblast of 

 the medullary canal. To this class belongs the retina, including 

 the pigment epithelium 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 an involution of the epiblast covering the 

 external surface of the embryo. These accordingly have no 

 primary connection with the brain. ' Taste bulbs ' and other 

 terminal nervous organs, such as those of the lateral line in 

 fishes, are also structures formed from the external epiblast. 



In addition to these we have the crystalline lens formed of 

 involuted epiblast as well as the cavity of the mouth and anus, 

 and the glands derived from them. The pituitary body is also 

 epiblastic in origin. 



From the hypoblast are derived the epithelium of the diges- 

 tive canal, the epithelium of the trachea, bronchial tubes and air 

 cells, the cylindrical epithelium of the ducts of the liver, 

 pancreas, thyroid body, and other glands of the alimentary 

 canal, as well as the hepatic cells constituting the parenchyma 

 of the liver, developed from the hypoblast cylinders given off 

 around the primary hepatic diverticula. 



B. III. 20 



