A somewhat unusual process of regeneration takes place when the jelly-fish, Gonionemus vertens, is cut into pieces. As first shown by Hargitt, the cut-edges come together and fuse, and the pieces
Fig. 39½.—A. Aboral view of Gonionemus vertens. A¹. Side view of same. Dotted line in each indicates where jelly-fish was cut into halves. B, B. New individual from a half. As seen from above and from the side. C, C¹. New individuals from a ¼ piece. As seen from above and from the side. D. New individual from a piece less than ¼. It contained a part of one of the radial canals. A new proboscis with mouth regenerated in all pieces, but no new canals or tentacles.
assume the form of a bell, but the missing parts are not replaced.[60] I have worked on the same form and obtained substantially the same results. If the jelly-fish is cut in two, as indicated by the dotted line in [Fig. 39½], A and A¹, each half closes in and assumes the form shown in B, B. Each new jelly-fish has only the two original radial canals that each half had when separated from the other. A faint line along the region of fusion of the pieces seems to represent a new radial canal,—it is not represented in the figures,—and each half-proboscis has completed itself. There are not formed any new tentacles, except perhaps one, or a few more, where the cut-edges meet. Thus there is actually very little regeneration, although the typical jelly-fish form is assumed by the half-piece. If a jelly-fish is cut into four pieces, each piece containing one of the radial canals, the pieces also assume the bell-like form, as shown in C, C¹. A new proboscis develops from the proximal end of the old radial canal, and since this end is often carried to one side during the closing in of the piece, the new proboscis lies not at the top of the sub-umbrella space, but, as seen in the figure, quite to one side. Pieces even smaller than these one-fourth jelly-fish will assume the bell-like form, especially if they contain a bit of the margin of the old bell and a part of one of the radial canals, as shown in [Fig. 39½], D. Although I have kept these partial medusæ for several weeks, and have fed them during this time, I have found that the missing organs do not come back. That these pieces do undergo a certain amount of regeneration is shown by the formation of a new proboscis, and, in certain cases, a new radial canal. Even the tentacles may be partially regenerated, as Hargitt has shown,—especially, as I have found, if the margin of the bell is cut off very near the base of the line of tentacles. Small knobs appear along the cut-edge, but the pieces die before regeneration goes very far. If, however, the margin is cut off in only one quadrant, new tentacles may be produced along the cut-edge.
CHAPTER VII
PHYSIOLOGICAL REGENERATION. REGENERATION AND GROWTH. DOUBLE STRUCTURES.
During the normal life of an individual many of the tissues of the body are being continuously renewed, or replaced at definite periods. The replacement of a part may go on by a process of continuous growth, such as takes place in the skin and nails of man, or the replacement may be abrupt, as when the feathers of a bird are moulted. It is the latter kind of process that is generally spoken of as physiological regeneration. In the same animal, however, certain organs may be continually worn away, and as slowly replaced, and other organs replaced only at regular intervals.
Bizozzero has made the following classification of the tissues of man, on the basis of their power of physiological regeneration. (1) Tissues made up of cells that multiply throughout life, as the parenchyma cells of those glands that form secretions of a definite morphological nature; the tissues of the testes, marrow; lymph glands, ovaries; the epithelium of certain tubular glands of the digestive tract and of the uterus; and the wax glands. (2) Tissues that increase in the number of their cells till birth, and only for a short time afterward, as the parenchyma of glands with fluid secretions, the tissues of the liver, kidney, pancreas, thyroid, connective tissue, and cartilage. (3) Tissues in which multiplication of cells takes place only at an early embryonic stage, as striated muscles and nerve tissues. In these there is no physiological regeneration.
There are many familiar cases of periodic loss of parts of the body. The hair of some mammals is shed in winter and in summer. Birds renew their feathers, as a rule, once a year. Snakes shed their skin from time to time. The antlers of deer are thrown off each year, and new ones formed accompanied by an increase in size and branching of the antlers. In other cases similar changes may be associated with certain stages in the life of the animal. The milk-teeth of the mammals are lost at definite periods, and new teeth acquired.[61] The larval exoskeleton of insects is thrown off at intervals, and after each moult the body increases in size; but after the pupa stage is passed and the imago formed, there is no further moulting. In the crustacea, on the other hand, the adult animals moult from time to time, and the upper limit of size is less well defined than in the insects. The larvæ also pass through a series of moults.
An interesting case of physiological regeneration has been described by Balbiani in a unicellular form, stentor. From time to time a new peristome appears along the side, moves forward and replaces the old peristome, that is absorbed as the new one comes into position. In other infusoria the peristome may be absorbed before encystment, and a new one appears when the animal emerges from the cyst. Schuberg states that when division takes place in bursaria the new peristome develops on the aboral piece in the same way as after encystment; and Gruber observed that, when an aboral piece of an infusorian is cut off, a new peristome develops in the same way as after normal division of the animal. These observations indicate that the process of physiological regeneration may follow the same course and probably involves the same factors as the process of restorative regeneration.