Small pieces of the liver were cut off and placed in the lymph gland. They did not always grow as well as did the preceding tissues, but often went to pieces. If they healed, the liver tissue often remained unchanged for several weeks. After two or three weeks connective tissue appeared between the peripheral liver cells, separating the cells from each other. The cells grew smaller, their protoplasm disappeared, and they at last disintegrated. Pieces of the gall duct behaved differently. They sometimes showed active growth, leading to the development of numerous branched canals.[86]
Pieces of the kidney, when transplanted, suffered a great change, and were subsequently absorbed. Transplanted pieces of a testis also changed. After six days, Sertoli’s cells and the spermatozoa disappeared. A kind of indifferent cell remained, characterized by clear protoplasm and by a large nucleus. After seventeen days further changes were observed, and later the pieces were completely absorbed. Pieces of the ovary rapidly disappeared, leaving only a mass of interstitial connective tissue.
The connective tissue underwent, in all the transplanted pieces, characteristic changes. The tissue became less dense, the protoplasm and nucleus of each cell enlarged. The cells multiplied, but only very slowly. These changes took place after one or two days. After a month or two the cells became more compact, their processes more numerous, and the nucleus small and long. Later degeneration set in.
Small pieces of bone from the caudal vertebræ were also transplanted, care being taken that each piece should contain some of the periosteum and marrow. The bone tissue goes to pieces, but the periosteum and marrow develop further. New bone is formed from the cells of the marrow as well as from those of the periosteum. Finally the entire piece, both its old and its new parts, is absorbed. Pieces of muscles were also absorbed.
These experiments of Ribbert show that transplanted pieces of tissue do not increase in size by growth, but undergo changes which he describes as a return to an earlier condition of development. The abnormal condition of their existence seems to be the cause of this change. The transformation may be due to a change of nourishment, or to a loss of nerve influence, or to lessened functional activity.
These results have a direct bearing on the problem of regeneration. They show that all kinds of tissue may continue to live, and the cells multiply in different parts of the body, but there seems to be nothing in these cases comparable to a regeneration of the entire organ. In the new situation the cells often assume an entirely new arrangement. After a period of activity, a process of degeneration commences, and the piece atrophies. Ribbert thinks that the atrophy is due to lack of nourishment, yet it is not clear how this could be the case, since for the first few weeks after transplantation there is an active growth, and in some cases, as in that of the bone, there is a formation of new, characteristic tissue. It may be that the transplanted tissues can no longer manufacture the substances necessary for their specific growth, and after the materials that have been brought along with them have been used up, the growth of the piece is stopped and its subsequent degeneration begins. It would be interesting to see if pieces transplanted to the same kind of organ as that to which they belong will become permanently incorporated in their new position.
The grafting-experiments that have been described in the preceding pages were carried out with pieces of adult organisms. Somewhat different conditions are present when parts of the developing egg or embryo are united, inasmuch as a process has been started in them that may go on independently, to a certain extent, of the union of the pieces. Born has carried out a large number of experiments in grafting parts of tadpoles of the same species, and also of different species. The union is brought about at the time when the tadpoles are about to leave the jelly membranes. The cut-surfaces are brought in contact and the pieces pushed together and held in place for an hour or two by means of small silver blocks or pieces of wire. The pieces readily stick together, and the union is a permanent one. Before describing Born’s results, it may be well to consider the power of regeneration of young tadpoles. If the tail is cut off a new one is regenerated by the tadpole, but all parts of the body do not have this same power. Schaper found that if a part of the brain, or even the entire brain, is removed, no regeneration takes place. I have found that if the region where the heart is about to develop is cut out from a young embryo, a new heart is not formed.[87] If a tadpole is cut in two across the middle of the body, neither piece regenerates the missing half. Byrnes has found, however, that if the region from which the posterior limb develops is cut out a new limb regenerates. In older tadpoles, Spallanzani found that if the hind limb is cut off it will regenerate, and Barfurth has more recently confirmed this result. The end of the tail that has been cut off from a young tadpole, before the tail has begun to differentiate, may continue alive for several days. It grows larger and flatter, and the V-shaped mesoblastic somites are formed. A slight regeneration even starts at its anterior end, as first observed by Vulpian and later by Born. The notochord and nerve-cord may send new tissue into the new part, and even some of the muscle cells may extend into this part, but the piece dies before regeneration goes any further. If, however, the tail is grafted in a reverse direction on the body of another tadpole, the regeneration may go further and produce a tail-like structure, as Harrison discovered and as I have also seen.
Born found that if the anterior half of one tadpole was united to the posterior half of the same or of another tadpole a single individual was formed which he kept alive in several cases until the time of metamorphosis. If the head of a tadpole is cut off and grafted upon the side of the body of another tadpole, the head will remain alive and continue to develop in its new position, and, if well nourished by means of the connecting blood vessels that develop, it may grow to be as large as the head of the tadpole to which it is attached. Similarly, if the tail of one tadpole is grafted upon the side of the body of another tadpole, it also continues to develop, and at the time of metamorphosis, when the normal tail is absorbed, the additional or misplaced tail also shows signs of breaking down. Even the posterior half of one tadpole, if grafted to the ventral side of another, may continue to develop, producing legs, etc.
Born succeeded in uniting tadpoles of different species in several different ways. They were united by their heads or by their ventral surfaces, or longer and shorter tadpoles made by using pieces longer or shorter than a half. In all of these cases there is no regeneration at the place of union, and the internal organ, the digestive tract, nervous system, and blood vessels unite when brought into contact. When pieces are united end to end, like organs unite to like, the nerve-cord with the nerve-cord, digestive tract with digestive tract, segmental duct with segmental duct, cœlom with cœlom, and although less often, the notochords sometimes join together. The lack of union of the ends of the notochord is explained by its frequent partial displacement at the cut-end, for when the cut is made the notochord, being tougher than the other structures, is often dragged out of place in one or in both pieces, so that the ends do not meet when the pieces are put together. When like organs are brought together the substance of one unites directly with the substance of the other, and if the organ is a hollow one, as is the digestive tract or the nerve-cord, their cavities also become continuous. There is also, Born states, some evidence to show that if similar organs are not brought exactly in contact their ends find each other and unite, and if they do not at first meet squarely they may do so later. When the ends of unlike organs are brought in contact, as, for instance, the nerve-cord and notochord, they do not unite, but connective tissue develops between them. The union of like parts, Born suggests, may be due to some sort of cytotropism, the outcome of a mutual attraction between similar cells like that which Roux has observed between the isolated cells of the segmented egg of the frog. Born thinks that the first rapid union of the pieces is due to the attraction of the ectoderm of one component for that of the other.
Born succeeded also in uniting pieces of the tadpoles of different