. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . e considered asone rendering thegerm plasm theoryabsolutely untenable,it is necessary toprove what the re-generated parts arecomposed of. Arethey composed sim-ply of repeated simi-lar cells, all of onetissue type, or dothey include otherkinds of cells or tis-sues than those par-ticular kinds fromwhich the regener-ated part springs?It is, of course, ad-mitted that many,indeed most cells ofthe body, can repro-duce other cells like themselves. Now is it a fa
. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . e considered asone rendering thegerm plasm theoryabsolutely untenable,it is necessary toprove what the re-generated parts arecomposed of. Arethey composed sim-ply of repeated simi-lar cells, all of onetissue type, or dothey include otherkinds of cells or tis-sues than those par-ticular kinds fromwhich the regener-ated part springs?It is, of course, ad-mitted that many,indeed most cells ofthe body, can repro-duce other cells like themselves. Now is it a fact that regen-erated parts are composed of different kinds of cells? As amatter of fact this has been proved to be so by observationand by experiment. Numerous instances are known in whichbody cells arising originally from one germ layer have pro-duced in the course of regeneration not only cells like them-selves, but others which in normal development could onlyarise from another germ layer. So it is plain that the studyof regeneration has already done much to modify our formerconceptions of the factors and conditions of FIG. 172.—Regeneration of the blastula and gastrulaeof sea urchins; line indicates where the blastula orgastrula was cut in half; the smaller figures show re-sults of the regeneration of the two halves of each. CHAPTER XIVPALEONTOLOGY This much then we have gained, that we may assert withouthesitation, that all the more perfect organic natures, such as fishes,amphibious animals, birds, mammals, and man at the head of the listwere all formed upon one original type which varies only more or lessin parts which are none the less permanent, and which still dailychanges and modifies its form by propagation.—Goethe (1796). IN a suggestive sentence, Haeckel speaks of our knowledgeof the line of descent in the history of any group of animals orplants as being derived from three ancestral documents-morphology, embryology, and paleontology. Of these thr
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