The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . dsthe rudimentary jaw-skeleton; the upper and lower jawswhich enclose the cavity of the mouth and carry theteeth. The Acrania and Monorhina are entirely destituteof these important parts. They first appear in the genuineFishes, and have been transmitted by these to the higherVertebrates. The original formation of the human mouth-skeleton, of the upper and lower jaws, can thus be tracedback to the earliest Fishes, from which we have inheritedthem. The teeth originate from the outer skin-covering
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . dsthe rudimentary jaw-skeleton; the upper and lower jawswhich enclose the cavity of the mouth and carry theteeth. The Acrania and Monorhina are entirely destituteof these important parts. They first appear in the genuineFishes, and have been transmitted by these to the higherVertebrates. The original formation of the human mouth-skeleton, of the upper and lower jaws, can thus be tracedback to the earliest Fishes, from which we have inheritedthem. The teeth originate from the outer skin-coveringwhich covers the jaws; for, as the formation of the wholemouth-cavity takes place from the outer germ-layer, the teethmust, of course, also have developed originally from the skin-layer. This can be actually proved by close microscopicexamination of the most delicate structural features of theteeth. The scales of Fishes, especially of Sharks, are, inthis respect, exactly similar to their teeth (Fig. 283). Thusthe human teeth, in their earliest origin^ are modified fish- 332 THE EVOLUTION OF scales.^^^ On similar grounds we must regard the salivaryglands, which open into the mouth-cavity, as really outer-skin (epidermic) glands, which have not developed, like theother intestinal glands, from the intestinal-glandular layerof the intestinal canal, but from the outer skin, from thehorn-plate of the outer germ-layer. It is evident that, asthe mouth develops in this way, the salivary glands mustbe placed genetically in the same series with the sweatysebaceous, and milk glands of the epidermis. The human intestinal canal istherefore quite as simple in itsoriginal formation as the primitiveintestine of the gastrula. It alsoresembles that of the lowest It then differentiates into two sec-tions, an anterior gill-intestine, anda posterior stomach-intestine, likethe intestinal canal of the Lanceletand the Ascidian. By the develop-ment of the jaws and gill-archesit is modified
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Keywords: ., bookcentury1800, booksubjectembryologyhu, booksubjecthumanbeings
