Fig. 51.—Two pieces of Bipalium kewense united by posterior ends. Each regenerated a head at anterior end.
A large number of experiments have been made by Joest in grafting pieces of earthworms. The cut-surfaces were held in contact by means of two or three threads passing through the body wall of each piece and tied across, so that the pieces were drawn together and held firmly in that position. Joest found that pieces of the same or of different individuals could be united in various ways, and the union become permanent. If the anterior end of one worm is united to the posterior end of the same, or of another worm, a perfect union is formed, and no subsequent regeneration takes place ([Fig. 52], A). Long worms can be made by uniting two pieces, each more than half the length of a worm, or by uniting three pieces, as shown in [Fig. 52], C. Short worms can be formed by cutting a middle piece from a worm, and uniting the anterior and posterior pieces ([Fig. 53], D). Joest found that when a short worm is made in this way, so that no reproductive region is present, the new worm does not produce new reproductive organs. It is conceivable that new reproductive organs might have been produced either in the old segments, or by the formation of a new reproductive region between the two united pieces, but neither process takes place. In the long worms two sets of reproductive organs, etc., are present. This sort of union is, however, less permanent, as the worms often pull apart.
Fig. 52.—After Joest. A. Union of two pieces of Allolobophora terrestris in normal position. Twenty-two months after operation. B. Union of two pieces Lumbricus rubellus. Pieces turned 180° with respect to each other. C. Union of three pieces of A. terrestris to make a “long worm.” D. Union of two worms (by anterior ends) from each of which eight anterior segments had been removed. After three months. Regenerating two new heads. E. A small piece of Lumbricus rubellus grafted upon Allolobophora terrestris. Former regenerated an anterior end.
Joest also united two posterior ends by their anterior surfaces. In many cases no regeneration took place, and, in the absence of a head, the combination is destined to die, although it may remain alive, without food, for several months. When two very long pieces were united by their anterior ends,—only eight segments being removed from each worm,—although perfect union took place at first, later one or two new heads generally developed at the region of union ([Fig. 52], D). When only one head developed it did not seem to belong to one of the components rather than to the other, and originated in the new tissue that appeared between the two pieces. These experiments, in which the anterior surfaces of two pieces are united, show also that the new head arises between the two pieces most often, if not exclusively, when the union is in the anterior ends of the worms. This corresponds with what is now known in regard to the development of new heads by isolated pieces, since there is less tendency to produce a head the farther posteriorly the cut has been made. At more posterior levels a tail and not a head is often regenerated, as has been stated, on the anterior cut-surface. This formation of a heteromorphic tail seems to have been suppressed in the pieces united in this region, except in one case,[82] in which it appears, from Joest’s account, that a tail probably regenerated, although Joest speaks of it as a head.
It is more difficult to unite two anterior ends by their posterior cut-surfaces, not because the surfaces refuse to unite, but because the two pieces crawl away from each other and pull apart. In one case, however, union of this sort was brought about.
In all the combinations that have been so far described, the dorsal and ventral surfaces of both components were kept in the same direction, so that the ventral nerve-cord of one piece came in contact and fused with the nerve-cord in the other piece. Sometimes it may happen that the components are not quite in the same position, and the end of one nerve-cord may fail to abut against the other one. In such cases Joest thinks that regeneration is more apt to take place in the region of union, and he has carried out a series of experiments in which the pieces were intentionally united, so that they are not in corresponding positions. It is found that if one piece is turned so that the nervous system lies 90 degrees, or even 180 degrees ([Fig. 52], B), from that of the other piece, the union takes place just as when the pieces have the same orientation, except that the ends of the nerve-cords do not unite. Subsequent regeneration from one or from both components generally takes place in the region of union.