. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . e of sea- urchin eggs and the con-sequent partial escapeor protrusion of parts ofthe egg plasm formingso - called extra - of these extra-ovates began develop-ment as a distinct bias-tula, the remainder ofthe egg forming anotherblastula (Fig. 163).Thus we see that experimental work has, so far, not afforded a positive answer to the general query proposed by the pre- formation versus epigen- esis problem. But at the same time it is obvious that
. Evolution and animal life; an elementary discussion of facts, processes, laws and theories relating to the life and evolution of animals . e of sea- urchin eggs and the con-sequent partial escapeor protrusion of parts ofthe egg plasm formingso - called extra - of these extra-ovates began develop-ment as a distinct bias-tula, the remainder ofthe egg forming anotherblastula (Fig. 163).Thus we see that experimental work has, so far, not afforded a positive answer to the general query proposed by the pre- formation versus epigen- esis problem. But at the same time it is obvious that the results of the experimental method are of extraor-dinary interest and of brilliant promise. What seems to be revealed so far, is that the animal egg is certainly not rigidly preformed; that there is no absolute predetermination of the fate in development of each part of the egg stuff. But that nor-mally in most eggs a given part of the egg does have a prospective definitive fate, so that one-half of the egg may be looked on as corresponding to one particular half of the future organism. However, the actual potentiality of any part. FIG. 162.—Cleavage of Echinus eggs in water freefrom calcium. Note that the cleavage cells tendto separate entirely. (After Herbst.) FACTORS IX ONTOGENY 281 of the egg is not limited by its prospective fate. If accidentin nature or ruthless handling in the experimenters labora-tory destroy or remove part of the egg, the remainder has apower of regulation which is in some respects the highestand most important kind of organic adaptation that we know. The same data derived from theexperimental study of development, to-gether with data got from the experi-mental study of mature and evensenescent stages of various organisms,constitute our chief evidence touchingthe problem of mechanism versus vi-talism. This problem may be posed inquestion form as follows: In how farcan so-called vital phenomena be ana-lyzed into physicochemical, or mechan-ical phe
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