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

FIG. 167.-Regeneration of the earthworm: A, Normal worm; BF, anterior ends of worms which, after the removal of one, two, three, four, and five segments, have regenerated the same number; G, anterior third cut off, only five head segments regener

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 connection with biological phenomena; the mechanists should have our strong sympathy 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 behind the middle, a heteromorphic tail regenerated at anterior end. (After Morgan.)

the more radical of them in their claim that the life mystery chemistry, that there

is already solved in terms of physics and 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

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 eggs of animal species in which parthenogenetic development does not occur naturally, or at least is very rare.

B

C

D

A

E

F

G

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 being looked on as perhaps the immediate stimulus. At present he seems inclined to attribute 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

FIG. 168.-Regeneration of the flatworm, Planaria lugubris: A, shows by dotted 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.)

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

FIG. 169.-Regeneration of the tail and limbs of the lizards, Lacerta agilis and Triton custatus: A, Lacerta, new tail arising at place where old tail was broken partly off; B, three-tailed form, two tails having a common covering, all these parts

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 animals. If Hydra, the common little brown or green fresh-water

being regenerated after old tail was cut off; C. polyp, be cut up into

Triton, additional leg produced by wounding femur;
D, double foot produced by tying thread over re-
generating stump; E, F, G, regenerated feet of
Triton after various mutilations. (After Tarnier.)

many pieces, each of these pieces has the 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 understood facts about regeneration are needed. And this need it seems to be the province of experimental biology to supply. By the carry

FIG. 170.-Regeneration of the flatworm, Planaria: A, Specimen cut in two as far forward as eyes, regenerating two half-heads; B, cut in two at one side of middle line, smaller piece having regenerated a head; C, cut partly in two, having regenerated two heads in angle; D, another that produced only a single head in the angle. (After Morgan.)

ing on of ingeniously planned and carefully controlled series

of experiments with regenerating 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 regeneration will throw light on one of the fundamental problems in development. To those biolo

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 regenerated part springs?

FIG. 172.-Regeneration of the blastula and gastrula It is, of course, ad

of sea urchins; line indicates where the blastula or gastrula was cut in half; the smaller figures show results of the regeneration of the two halves of each.

mitted that many,

indeed most cells of the body, can repro

duce 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.