Fig. 55.—A. Rana sylvatica with grafted tail of Rana palustris. Line a-a indicates where tail was cut off. B. Rana palustris with grafted tail of Rana sylvatica. Line a-a indicates where tail was cut off. C. Older stage of a graft like B. Lines indicating two possible operations. D. Another individual with two tails, one composed of both components. E. Later stage of last, when tail was cut off at level a-a.
Another series of experiments in grafting, similar to one of those made by Joest and myself on the earthworm, has been made by Harrison on the tadpole. I have also later made similar experiments. Two tadpoles are united by their posterior ends, as shown in [Fig. 54], A, and a day or two after union one of the tails is cut off near the line of union. There is thus left attached to the end of the tail of one tadpole a part of the tail of the other united in a reverse direction, so that the exposed cut-end is the anterior end of the small piece. There grows out from this cut-end a structure that resembles a tail ([Fig. 54], B, C, D). It contains a continuation of the notochord and nerve-cord, that taper in a characteristic way to the end of the new structure. The tail is flat and has a central band of muscle tissue, and a dorsal and ventral fin. The muscles of the normal tail have a characteristic V-shaped arrangement with the apex of the V’s turned forward, but unfortunately in the new tail the muscles are so irregular that it is impossible to make out their arrangement ([Fig. 54], D). If the new part is in reality a tail, the V’s ought to stand in the same way as do those in the major component, and opposed to the V’s on the part from which the new material arises. If the new structure is not a tail at all, but a new growth, or even a suppressed trunk, then the V’s should stand as in the small part itself. It has not been possible as yet to obtain a decisive case. Harrison obtained one case in which the arrangement of the muscles in the new part seemed to be more as it should appear if the new part is a heteromorphic tail ([Fig. 54], D). Even if this could be shown to be the case, it may be that under the conditions of the experiment the arrangement of the muscles is determined by the use of the tail, although this does not seem very probable. Harrison, after a careful analysis of the question, left it undecided, but seemed more inclined to the view that the result is due to the development of something new rather than a heteromorphic growth. On the contrary I am strongly inclined to believe that the latter is the true explanation. In another way I have been able to bring about the development of the same structure. A small triangular piece is cut from the upper part of the tail, as indicated in [Fig. 56], A, one point of the triangle passing through the notochord, or even through the aorta. If the cut-surfaces are kept apart for a few hours, until the exposed end has been covered over by ectoderm, they may not unite afterward, and two exposed surfaces are left,—one at the distal end of the base of the tail, and the other at the proximal end of the outer part of the tail. The latter surface corresponds to that in the grafting-experiment. Regeneration may take place from the two surfaces; both new parts seem to be exactly alike, and both resemble a regenerated tail. The one from the proximal surface of the outer part of the tail contains a notochord, nerve-cord, connective tissue, pigment cells, and muscle tissue ([Fig. 56], B). The arrangement of the muscle fibres is generally very irregular, and the characteristic V-shaped arrangement cannot be detected.
In only a few cases have attempts been made to unite two eggs or two very early embryos, although there are a few casual observations[89] in which such a fusion has been observed. The problems that arise in connection with the union of two eggs are full of interest. Each egg has the power of producing an embryo of normal size. If two eggs are united into one, will a single giant organism result, or two organisms? If the former, we must suppose that a new organization is formed of double size. Whether an upper limit of organization exists can only be determined by such an experiment. If two fused organisms result from the fusion of two eggs, it would show the structure of the egg is of such a kind that two organizations cannot readjust themselves into a single one of double size. Moreover, it is important to discover whether any difference exists as to the stage of development at which the union is brought about, for it is conceivable that while a rearrangement is possible at one stage, it might not be at another.
Fig. 56.—A. Tadpole to show where the V-shaped piece is cut from the tail. B. Later stage of same with a new tail-like outgrowth from the anterior end of tail.
It has been shown that two blastulæ of the sea-urchin can be united to form a single embryo. I found (’95) that occasionally two blastulæ stick together and fuse, so that a single sphere of double size is formed. As a rule two gastrulæ and two more or less complete embryos develop from each double blastula, but in a few cases I found that a single embryo may be formed, that shows, however, traces of its double origin. Driesch has more recently (1900) succeeded[90] in bringing about more readily a union of two segmenting eggs or blastulæ, and obtained perfect single individuals from two fused blastulæ. He finds that if the fusion takes place at an early stage the resulting embryo is less likely to show its double origin than when older blastula stages are united. Zur Strassen has also observed giant embryos of ascaris that arise by a fusion of two eggs. Loeb has found that the eggs of chætopterus, which can be made to develop parthenogenetically in certain salt solutions, often stick together and produce giant embryos.
CHAPTER X
THE ORIGIN OF NEW CELLS AND TISSUES
There are many difficulties in the way of determining the origin of the cells that make up the new part. The only means at present at our command for studying their source is by serial sections of a number of different stages taken at intervals from different animals. Since there may be differences between the processes in different individuals, and since we can only piece together the information gained from successive stages, much uncertainty exists in regard to the changes that take place during regeneration, even in some of those forms that have been examined over and over again. Were it possible actually to follow out the movements of the living cells in one and the same animal, the problem would offer fewer difficulties, but this cannot be done. It will be more profitable to consider first the better-known and simpler processes, and afterward those that are less well-known.
The regeneration of the head and tail of lumbriculus and of certain naids is a comparatively simple process, and has been studied by several investigators, whose results agree, at least in regard to the most essential features. Semper (’76) described the origin of the new organs in the formation of new individuals by budding in nais. He found that the new brain and nerve-cord develop from the ectoderm, the new mesoderm also from ectoderm, and the new digestive tract from the old one, except the pharynx, which arises by the fusion of two mesodermal “gill-slits.” Bülow (’83) studied the regeneration of the tail of lumbriculus. He found the ventral cord in the new part arising from a paired ectodermal thickening, the mesoderm arising from a proliferation of cells. These cells are invaginated in the region between ectoderm and endoderm—the in-turning of the proctodæum being looked upon as an endodermal invagination.[91] The more recent work of Randolph, Rievel, Michel, Hasse, Hepke, and von Wagner on the same or related forms has served to point out certain errors in the earlier work of Semper and Bülow, and has added some new and important facts, especially in connection with the origin of the mesoderm in the new part. Without attempting to give a detailed account of these results,