Chapter XIV: General Considerations and Conclusions (3)

[59] _Über Endothelwucherungen in Arterien. Beitr. z. pathol. Anat._, Vol. VIII, 1890.

[60] Haeckel (1870) first showed, in another medusa, that pieces produce new medusæ.

[61] In rodents, however, the incisors continue to grow throughout the life of the animal.

[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_).

[93] This seems to be true for urodeles, but whether it is true for the anurans is rather a question of definition, as I have pointed out in my book on _The Development of the Frog’s Egg_.

[94] The attachments of the muscles may be the cause of the break in the middle of the vertebræ, rather than between two vertebræ.

[95] _Prodromo_, 1768.

[96] Philipeaux, _Comptes rendus de l’Acad. des sciences de l’Institut de France_, Année 1866, 1867.

[97] Todd (_Quarterly Journal of Science, Literature, and Arts_, Vol. XVI), Blumenbach, Treviranus, Von Siebold.

[98] How the tentacles could have gotten into their normal position is not explained.

[99] The foot sometimes pushes out through one of the slits made by the bristle instead of out of the mouth.

[100] I have given elsewhere (_The International Monthly_, March, 1901) a fuller treatment of the gastræa theory from the historical point of view.

[101] It may be pointed out that there may be really several kinds of homology, such as homology due to similar origin of the blastomeres, or to their position, or to their fate, etc. The confusion that has arisen may in part result from the attempt to make homologous parts agree in all points.

[102] That is, one not depending on inheritance through adult forms.

[103] _Biologisches Centralblatt_, XV, ’95.

[104] A small amount of embryonic mesenchyme may come from some of the ectodermal quartettes of the embryo and produce the branching muscles of the head, but not the characteristic muscles of the trunk.

[105] Cosmos, Vol. VII, p. 388.

[106] King pointed out the fallacy of this argument.

[107] Roux’s earlier experiments in 1885, in which the unsegmented or segmented egg was stuck and a part of its contents removed, the remaining part making a whole embryo, will be considered in another connection.

[108] This had been first discovered by Newport in 1851.

[109] The cross-section _C_ is reversed as compared with the half-embryo _B_.

[110] This difference is due, I suppose, to the amount of injury that the nucleus of the injured half may have suffered.

[111] The development of isolated blastomeres of the ctenophore egg shows that this need not be the case.

[112] In one case a half-embryo resulted.

[113] The plane of the first cleavage has been shown in two urodeles to correspond, not to the median longitudinal plane, but to a cross-plane of the embryo.

[114] In some cases, especially in sphærechinus, even at the eight-celled stage, the blastomeres seem to shift their position, so that a whole sphere of half size is formed.

[115] Hertwig had a year before expressed a similar view in regard to the equivalency of the blastomeres.

[116] A view advanced by Pflüger.

[117] The evidence to show that more than four and certainly more than eight such groups that come from a single egg can produce a pluteus is, I think, insufficient, and the result improbable.

[118] Driesch’s figures seem to show, nevertheless, that the archenterons are proportionately too large.

[119] These may be pieces that were cut obliquely, as Driesch suggests, so that they contain a part of the archenteric region.

[120] Driesch, Hertwig, Roux, Weismann, Barfurth. For review see Driesch (’95).

[121] Bunting (’94) also found that isolated blastomeres of hydractinia make whole embryos.

[122] If the yolk of the dividing egg is partially withdrawn without disturbing the blastomeres, the form of the cleavage may be altered, but a normal whole embryo develops over the smaller yolk sphere.

[123] We offered as a possible explanation in this case that the egg had been cut in two symmetrically with reference to the eccentric nucleus.

[124] These experiments have been quite fully described in my book on _The Development of the Frog’s Egg_.

[125] Not, however, the supposed action of gravity on the egg.

[126] As stated in my article on “The Problem of Development,” 1900.

[127] According to Roux.

[128] According to E. B. Wilson.

[129] 1897.

[130] Unless it produces a physical change in the structure.

[131] Stevens (’01) has found that this ball of red pigment is ejected from the mouth of the new hydranth.

[132] The importance of this conception is, in my opinion, marred by the fiction of the ferment action of the nucleus; but it should not be overlooked that Driesch avowedly called this a pure fiction.

[133] Not that Driesch supposes this would be the case.