262 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



transplantability in these early embryonal states is, therefore, still greater 

 than in the later ones. 



While it was possible to keep embryonal tissues alive, at least for a short 

 time, after hetero- or xenotransplantation and to obtain organizer effects, 

 it has been found more recently that it is not necessary to transplant living 

 tissue in order to obtain induction. In these experiments the dorsal lip of the 

 blastopore from urodele gastrulae, which had been found to contain especially 

 efficient organizer material, was used primarily, but also other material, such 

 as medullary plate, with the underlying mesodermal structures, showed marked 

 organizer effects. In contact with ectoderm of the gastrula, this material 

 caused the transformation of the ectoderm into a medullary plate. Thus 

 Spemann, Bautzmann, Holtfreter and Mangold could show that tissue which 

 had been killed by exposure to heat or cold, by drying or by mechanical 

 means, could still function as an active organizer. Likewise tissues treated 

 with alcohol, ether, acetone, glacial acetic, hydrochloric acid or xylol, or 

 infiltrated with paraffine, were still effective. Moreover, tissues which, in the 

 living state, lacked the ability to act as inductors, acquired this property after 

 they had been killed by drying (Holtfreter), or following treatment with 

 acetone and alcohol. Thus entoderm or ectoderm of gastrulae acquired the 

 ability to induce medullary plate formation after they had been exposed to 

 such treatment. Even the unsegmented egg could, under these conditions, 

 induce the development of very differentiated organs, such as the lens of the 

 eye, whole eyes, optic vesicles, and parts of the brain. Spemann has suggested 

 that the manifestation of organizer action in material formerly devoid of 

 such effects may be due to the removal, by means of solvents, of inhibiting 

 substances which had been present in the living, inactive tissues, or the 

 changes which take place during the denaturation of protein may set free 

 the active organizer. As to the chemical nature of the substances which act 

 as organizers, the evidence obtained so far appears to be contradictory; the 

 effects have been attributed to various substances, glycogen, proteins, and 

 simpler hydrocarbons. It is possible that proteins in combination with glycogen 

 or with certain non-specific lipoid substances may act as organizers ; also 

 estrogens and carcinogenic hydrocarbons may perhaps exert organizer func- 

 tion (Needham and Waddington). Some investigators have found that injury 

 of embryonal tissue may activate the organizer. 



Considering the fact that dead tissues of amphibian embryos and certain 

 extracts from such tissues may serve as organizers, it is not surprising to learn 

 further that a great number of organs of embryos or adult forms of verte- 

 brates as well as of invertebrates may act as organizers in gastrulae of Triton. 

 In some instances adult tissue from distant species has first to be killed by 

 heat before it will thus act ; but in other cases, strange adult living tissues 

 exert this function after transplantation. 



These results apparently are contradictory to the great specificity of the 

 factors which are evident during embryonal life, where only definite organs 

 and not others can act as organizers at given periods of embryonal develop- 

 ment and in definite areas of the embryo if certain results are to be obtained. 



