332 



THE EVOLUTION OF MAN. 



On similar grounds we must regard the salivary 

 glands, which open into the mouth-cavity, as really outer- 

 skin (epidermic) glands, which have not developed, like the 

 other intestinal glands, from the intestinal-glandular layer 

 of the intestinal canal, but from the outer skin, from the 

 horn-plate of the outer germ-layer. It is evident that, as 

 the mouth develops in this way, the salivary glands must 

 be placed genetically in the same series with the sweat, 

 sebaceous, and milk glands of the epidermis. 



The human intestinal canal is 

 therefore quite as simple in its 

 original formation as the primitive 

 intestine of the gastrula. It also 

 resembles that of the lowest Worms. 

 I It then differentiates into two sec- 

 j$fo tions, an anterior gill-intestine, and 

 ft a posterior stomach-intestine, like 

 li the intestinal canal of the Lancelet 

 and the Ascidian. By the develop- 

 ment of the jaws and gill-arches 

 it is modified into a true Fish- 

 intestine. Afterwards, however, the 

 gill-intestine, which is a memorial 

 of the Fish-ancestors, as such, is 

 entirely lost. The parts that remain 



Shark (Centrcphorus calceus). take a wholly different form ; but 

 On each rhomboid bone-tablet, notwithstanding that the anterior 



lying in the leather-skin, rises 



a small, three-cornered tooth, section of our intestinal canal thus 

 (After Gegenbaur.) surrenders entirely its original 



form of gill-intestine, it yet retains its physiological func- 

 tion as a respiratory intestine; for the extremely in- 



FIG. 283. Scales 



