COMPARISON OF THE GERMINAL LAYERS. 253 



epiblast and the mesoblast coincides with that between the epi- 

 dermis 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 connec- 

 tion 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 invo- 

 luted 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 digestive 

 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. 



Homologous probably with the hepatic cells, and equally of hypo- 

 blastic origin, are the spheroidal 'secreting cells' of the pancreas and 

 other glands. The epithelium of the salivary glands, though these so 

 closely resemble the pancreas, is probably of epiblastic origin, inas- 

 much as the cavity of the mouth is entirely lined b)y epiblast. 



The hypoblast also lines the allantois. To these parts must be 

 added the notochord and subnotochordal rod. From 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. 



The generative and urinary organs are entirely derived from the 

 mesoblast. It is worthy of notice that the epithelium of the urinary 

 glands, though resembling the hypoblastic epithelium of the alimen- 

 tary canal, is undoubtedly mesoblastic. 



From the mesoblast are lastly derived all the muscular, connective 

 tissue, and vascular elements, as well of the alimentary canal and its 

 appendages as of the skin and the tegumentary organs. Just as it 

 is only the epidermic moiety of the latter which is derived from the 



