Fig. 57.—After Hasse. Regeneration of head of Tubifex rivulorum. A. Sagittal section of anterior end. Six days after cutting in two. B. Eleven days after cutting in two. C. Cross-section through new part. Five days after operation. D. Fourteen days after operation. E. Sixteen days after operation.
I shall describe the principal changes that have been found to take place. When the anterior end of lumbriculus or of tubifex is cut off, the cut-surface very quickly closes, as a result of the contraction of the body wall. According to some investigators, the circular muscles are chiefly concerned in the closing, but according to others the longitudinal muscles bring about the result. The cut-end of the digestive tract is pulled a little inward, and its end also closes ([Fig. 57], A). For a day or two no important changes can be observed to take place, but new ectoderm soon appears over the cut-surface. This ectoderm arises in all cases from the old ectoderm, and as it increases in amount the old ectoderm is pushed back from over the cut-end, leaving a layer composed of a single row of cells over the end. Since nuclei in process of division are rarely present before these initial processes begin, it is probable that the changes are due, in large part, to an out-wandering of ectodermal cells, or, what amounts to the same thing, to the leaving behind of cells as the old ectoderm withdraws from the cut-end. In the new ectoderm over the end, an active process of proliferation takes place ([Fig. 57], B), that leads to the production of a large number of cells lying within the new part. The ectoderm has at this time begun to bulge outward, so that the proliferated cells come to lie within the dome-shaped beginning of the new head. There appears to be some difference in the number and in the location of the proliferations in different species. In general, the new cells arise from the ventral and ventro-anterior region of the dome-shaped ectodermal covering of the new part. Most of this new material gives rise to the brain, commissures, and ventral nerve-cord ([Fig. 57], C). The cells giving rise to these structures in tubifex come from two ventral regions of proliferation that extend along the sides and dorsally to the anterior end in front of the digestive tract. Where the two masses meet above and in front, the brain is formed.[92] The cells that do not take part in the formation of the nervous system give rise to the muscles and connective tissue of the new head. These cells lie especially at the outer sides of the proliferated mass. The origin of the new muscles from ectoderm stands in sharp contrast to the current ideas in regard to the origin of new tissues, and yet it is a point on which the more recent investigators are entirely in accord. Michel, Hepke, and von Wagner have arrived at the same conclusion after a careful examination, and there seems to be no reason for refusing to accept their results. The theoretical importance of this discovery will be discussed later.
Soon after the proliferation from the ectoderm has begun, the blind end of the digestive tract starts to push forward ([Fig. 57], D). The cells in the most anterior part of its wall begin to divide, and the end grows in an anterior direction as a more or less solid rod. This rod extends, in some species, as far forward as the ectoderm, meeting the latter on the inner side of its antero-ventral surface. At this point an in-turning of ectodermal cells, in the form of a blind pit, develops, and later this pit, deepening to become a tube, forms the mouth cavity. Its inner end is from the beginning in contact with the anterior end of the digestive tract, or else it connects with the latter soon after its formation. The two flatten against each other, the cells draw away in the middle of the region of contact, and the cavity of the new mouth becomes continuous with the cavity of the old digestive tract. The mouth lies at first nearly terminal in position ([Fig. 57], E), but by the forward growth of the body wall over and in front of the mouth to form the prostomium, the mouth comes later to lie more on the ventral surface. The short tube produced by the in-turned ectoderm forms only a short part of the digestive tract. It leads from the mouth opening to the new pharynx, and forms, therefore, only the buccal cavity. A similar ectodermal tube, the stomodæum, which develops in the egg-embryo, becomes not only the buccal chamber, but also the lining of the pharynx. The latter is, therefore, considered an ectodermal structure in the embryo. On the other hand, in the regenerated head the lining of the new pharynx arises from the anterior part of the endodermal digestive tract. We find, therefore, that the same organ, the pharynx, may arise in the same animal from distinct “germ-layers.” This result also has an important bearing on our ideas concerning the value and meaning of the so-called “germ-layers,” and has helped to bring about a revolution of current opinion as to the importance of these layers.
The preceding account of the development of the head has shown that while certain of the new organs and layers arise from the same organs of the old part, yet this is not true for all of them. Thus while the ectoderm gives rise to ectoderm, the new muscles do not appear to come from the old ones, or even from other mesodermal tissues, but from the ectoderm. The old digestive tract gives rise to the greater part of the new one, but the new pharynx comes from the old endoderm, and not from the in-turned ectoderm. The nervous system does not arise from the old ventral cord, but from a proliferation of ectoderm. It has, thus, the same origin as the nervous system of the embryo. The origin of the new blood vessels has not been satisfactorily made out. The seta sacs arise from ectodermal pits as in the embryo.
In regard to the origin of the new mesoderm, the evidence is still insufficient, I think, to show that cells derived from the old muscles or peritoneum take no part in the formation of the new muscles and peritoneum; but that the greater part of the new muscles, etc., comes from the proliferated cells can scarcely be doubted. This latter discovery loses none of its significance, however, even if it should prove true that the old muscles, etc., contribute something to the new part. It is also not entirely disproven that the ventral nerve-cord does not take a small share in the development of the new cord.
The regeneration of a new tail-end in these same forms appears to take place in much the same way as the head. The cut-end quickly closes; later a layer of ectoderm appears over the posterior surface, and the new part bulges out and becomes dome-shaped. A paired, or in some species a single, region of proliferation develops from the ectoderm, that gives rise to the new ventral nerve-cord. Lateral proliferations of ectoderm produce, according to some writers, the material out of which the mesoderm of the new tail is formed. Randolph, on the other hand, has described the new mesoderm as arising from the old, especially from certain large peritoneal cells that are found throughout the body. The cut-end of the digestive tract closes, and later new cells develop at its posterior end. An in-turning of ectoderm, in the form of a pit, fuses with the posterior end of the digestive tract and establishes communication with the outside.
Fig 58.—After Hescheler. Regeneration of anterior end of earthworm. A. After four days. B. After eleven days. C. After twenty-five days. D. After twenty-one days (younger individual).