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been done to test the truth of the mechanical assumptions is a fascinating chapter in the history of modern biological work.
In sum we may say that there has been in recent years a real advance on a basis of experimental work, in the analysis of many vital phenomena long considered mysterious, or at least too complex for human understanding, into simpler components. And that these components are in many cases
no other than reactions and motions familiar to us in inorganic nature. On the other hand it must be said that this advance, in the face of the immense problem presented by vital reactionsthat is, the behavior of organisms—is very small. With all our heart we should welcome all attempts to do away with ideas of mysticism in con
nection with biologiFIG. 167.-Regeneration of the earthworm: A, Nor- cul phenomena; the mal worm; B-F, anterior ends of worms which, after mechanists should the removal of one, two, three, four, and five seg
have our strong symterior third cut off, only five head segments regener
pathy and our willing ated; H, worm cut in two in middle, a head-end of support, but to join five segments regenerated; I, worm cut in two be
the more radical of hind the middle, a heteromorphic tail regenerated at anterior end. (After Morgan.)
them in their claim
that the life mystery is already solved in terms of physics and chemistry, that there is no longer any vital problem, would be to surrender our judgment to our inclination.
Any discussion, however brief, of experimental work in biology should include a reference, at least, to the striking and suggestive results that have been obtained by the application of the experimental method to the investigation of the problems
ments. have regenerated the same number: G. an
of fertilization and parthenogenesis. Jacques Loeb has been the most active worker in this line and his results are of extreme interest. He has, by various physical or chemical treatment of the unfertilized eggs of various animals, particularly certain Echinoderms, worms and fishes, stimulated these eggs to begin development, which development proceeds either normally or in some degree abnormally along the usual path regularly followed by the species. But in all cases this development falls short of completion and in many cases the death of the embryo occurs at a very early stage. Other investigators
have similarly induced a de-
velopment in parthenogenetic C Fig. 168.-Regeneration of the flatworm,
eggs of animal species in which parthenogenetic develD F G Planaria lugubris: A, shows by dotted
opment does not occur naturally, or at least is very rare.
The significance of these results is by no means wholly clear. Nor do the investigators who have done the work agree among themselves as to the interpretation of the results. Loeb first inclined to the belief that the stimuli which incited the unfertilized egg to development were physical, osmotic changes be
ing looked on as perhaps the immediate stimulus. At present he seems inclined to at
line where the worm was cut in two lengthwise; B, C, D, show how a half that was fed regenerated ; E, F, G, show how an unfed half regenerated. (After Morgan.)
tribute the stimuli rather to
the chemical character of the media which seem to incite the parthenogenetic development. In either case the physicochemical stimulus is considered to be a substitute for the spermatozoid. That it is a substitute in some degree, is obvious; that it is a complete substitute for it, seems equally obviously not true. The embryos developed by artificial partheogenesis lack at least two fundamentally important attributes which the young of bisexual parentage possess; namely, vigor and the heredity of the father. The lack of vigor is shown by their death before maturity; and the chromosomes
or other nuclear stuff that is the actual carrier of the paternal heredity are of course actually wanting.
Another phenomenon or group of phenomena, also of much special interest and suggestiveness to students of development, to which the experimental method has been successfully applied, is that known as “regeneration.” The familiar repro
duction or growth of new plants from cuttings or buds is paralleled in the animal world by numerous similar cases less familiar but nevertheless long known by naturalists. In 1740, Abbé Trembley made a number of curious experiments with Hydra, whose publication in 1744 was the beginning of our knowledge of the phenomena of
regeneration in aniFig. 169.—Regeneration of the tail and limbs of
the lizards. Lacerta agilis and Triton cu status: mals. If Hydra, the A, Lacerta, new tail arising at place where old tail common little brown was broken partly off; B, three-tailed form, two
or green fresh-water tails having a common covering, all these parts being regenerated after old tail was cut off ; C, polyp, be cut up into Triton, additional leg produced by wounding femur; many pieces, each of D, double foot produced by tying thread over re
i these pieces has the generating stump; E, F, G, regenerated feet of " Triton after various mutilations. (After Tarnier.) power to grow into a
new complete Hydra body (Fig. 164). We know now that numerous other animals have also this radical capacity for regeneration. Certain protozoans, hydroids, planarian worms, starfishes, etc., can regenerate as freely or nearly so as Hydra (Figs. 165–172). And many other animals representing almost all the great groups of the animal kingdom possess in some degree, at least, the power of regeneration. Some can regenerate only lost or cut appendages, others even less fundamental parts of the body; some can regenerate only in their immature stages; others only in the earliest embryonic stages. But regeneration and "regula
tion," as certain phases of regeneration are called, are the property, in some degree probably, of most animals.
The significance of this capacity has been long recognized as of much importance in our conceptions of the germ plasm character and disposition, but no general agreement regarding it has even yet been reached by biologists. More and better under
Fig. 170.-Regeneration of the flatworm, Planaria: stood facts about re A, Specimen cut in two as far forward as eyes, generation are needed.
regenerating two half-heads; B, cut in two at
one side of middle line, smaller piece having reAnd this need it seems
generated a head ; C, cut partly in two, having to be the province of regenerated two heads in angle; D, another experimental biology to
that produced only a single head in the angle.
(After Morgan.) supply. By the carrying on of ingeniously planned and carefully controlled series
of experiments with re
generating animals, we
are acquiring a great mass of important data, and the interpretation and generalization of these data is certain to be accomplished in the near future.
We have space here to call attention to but one of the ways in which an understanding of the phenomena of regener
ation will throw light on Fig. 171.- Regeneration of the eye of Triton: one of the fundamental A, Edge of iris with beginning lens; B, C, D,
problems in developlater stages of same ; E, whole eve with regenerating lens. (After Wolff and Fischel )
ment. To those biolo
prove what the re
gists who believe with Weismann that there is a sharp distinction between the germ plasm and the somatic or body plasm, and that this germ plasm is limited to the germ cells and germ-cell producing tracts, the regeneration of a nearly whole body or even a considerable part of a body from a region which does not include a germ cell presents a serious obstacle. But
before this obstacle can be considered as one rendering the germ plasm theory absolutely untenable, it is necessary to prove what the regenerated parts are composed of. Are they composed simply of repeated similar cells, all of one tissue type, or do they include other kinds of cells or tissues than those particular kinds from which the regener
ated part springs? FIG. 172.-Regeneration of the blastula and gastrulæ It is, of course, ad
of sea urchins; line indicates where the blastula or mitted that many. gastrula was cut in half; the smaller figures show results of the regeneration of the two halves of each. indeed most cells of
the body, can reproduce other cells like themselves. Now is it a fact that regenerated parts are composed of different kinds of cells? As a matter of fact this has been proved to be so by observation and by experiment. Numerous instances are known in which body cells arising originally from one germ layer have produced in the course of regeneration not only cells like themselves, but others which in normal development could only arise from another germ layer. So it is plain that the study of regeneration has already done much to modify our former conceptions of the factors and conditions of development.
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