METHODS OF PHYSIOLOGICAL ANALYSIS 83 



EXPLANTATION 



As the term is usually employed, explantation is merely isolation of a 

 part in a special extraorganismic environment. This environment may be 

 supposedly indifferent : a balanced salt solution such as the various con- 

 centrations and modifications of Ringer solution adjusted to the organisms 

 concerned. If properly adjusted, such a medium prevents the loss of elec- 

 trolytes which may occur in water, particularly if isolation involves a 

 wound. Cultures in vitro of living cells, cell masses, organ primordia or or- 

 gans in special media, plasmas, and gels (nutritive, growth-stimulating, 

 etc.) are explantations. These experiments involve not only the effect of 

 isolation but also that of the medium on the isolated part. Explantation 

 methods have been extensively used in the culture of particular cells or 

 tissues of embryonic or later stages, but recently culture in vitro of larger 

 parts of embryos, chiefly of vertebrates, has been undertaken in the at- 

 tempt to provide environments favorable to further development. 



TRANSPLANTATION OR GRAFTING 



Transplantation consists essentially in altering the organismic environ- 

 ment of the part concerned. This can be accomplished in many ways: 

 transplantation may be autoplastic, to the same individual, homoplastic, 

 to another individual of the same species, heteroplastic, to another species 

 of the same genus, xenoplastic, to an individual of another genus, family, 

 order, etc. In any of these relations it may be orthotopic, to the position 

 originally occupied by the part concerned, or heterotopic, to some other 

 position. The part may be transplanted in normal orientation to the axi- 

 ate pattern of the host or in some other orientation. Donor, transplant, 

 or host may be subjected to experimental conditions before or after trans- 

 plantation. Size of transplant and developmental stage of donor or host 

 can be varied. Transplantation to the extraembryonic membranes of 

 birds is possible; this environment is still physiological but more or less 

 isolated from the axiate pattern of the host. Up to the present the most 

 extensive transplantation experiments with embryonic stages have been 

 made with amphibian and avian material, but transplantations of parts of 

 fully differentiated hydroids, planarians, and annelids have given results 

 of great value for analysis of developmental pattern. 



FUSION OF INDIVIDUALS 



With some organisms it has been possible to unite developmental stages 

 of two intact individuals. Here the questions are whether they can recon- 



