[62] If the young worm is fed the new part becomes almost as broad as the old piece, but if the worm is not fed the old part decreases in breadth and the new part does not grow as broad as in the former case. [63] See Fraisse for literature. [64] In the figure one double or forked toe is present. [65] A parallel case is found when a piece partially split in two at the anterior end (Fig. 24) produces one or two heads on each half, according to the extent of fusion of the new material that goes to form the new head or heads. [66] See Lang (’88). [67] See Zacharias (’86). [68] See Hescheler (’97). [69] The proglottids of the cestodes seem to be an exception, but they are little more than sacs filled with embryos at the time of their separation. How far regeneration may take place in the scolex, or young proglottids, is not known, but it is not improbable that some of the abnormal forms that have been described may be due to regeneration. [70] Except for the protozoa. [71] _The Fisheries and Fishing Industries of the United States_, Washington, 1887. [72] “The American Lobster,” _Bull. U. S. Fish Comm._, 1895. [73] Réaumur in 1742 records the first observations. Spallanzani also described the process, and many later writers have examined it. [74] The phenomenon has been observed by Dalyell, Semper, Minchin, and others. [75] Müller, _Elements of Physiology_, 1837. [76] By Wagner (’87). [77] For references to the literature on grafting in plants see Vöchting (’84). [78] In one case they separated only after three months. [79] This and other experiments were carried out by pushing the pieces on a bristle. [80] Rand found that when a posterior piece was grafted by its cut, oral end to the side of another hydra that it was absorbed into the stock. In one case it moved down the whole length of the body of the stock and finally disappeared by absorption into the foot of the stock. [81] Pieces from male and female colonies of the same species also unite. [82] See Joest’s Fig. 14. [83] It is not certain whether this is a head or a tail. [84] Joest states that this new part is a head, as shown by the presence of food matter in the digestive tract of the posterior piece. [85] The prostomium was misshapen, so that its specific character could not be made out. [86] It is known that the process of regeneration of the liver takes place especially from the gall ducts. [87] In one case I observed rhythmic pulsations in a vessel on one side of the neck, in the region above the pharynx. [88] The figure was drawn fifteen days after union. [89] Metschnikoff (’86), Herbst (’92). [90] Eggs without membranes were placed in sea water without calcium, to which a few drops of sodium hydroxide have been added. [91] The usual interpretation at present is to regard the proctodæal ingrowth as ectodermal. [92] In some species the two proliferating regions seem to be in contact above from the beginning (Hepke, in _Nais_).