CERTAIN GENERAL CHARACTERISTICS 31 



into a hydranth or the distal part of a hydranth by reorganization or re- 

 differentiation without any ''replacement of missing parts" by outgrowth 

 of new tissue. 



Regeneration is reconstitution by outgrowth of new tissue from the cut 

 surface and its differentiation into a part of an individual, often, but by no 

 means always, more or less similar to the part removed. Regeneration, as 

 defined, occurs widely in animals but is not common among plants, 

 though some algae and fungi regenerate and parts of vegetative tips re- 

 moved may be regenerated in some of the higher plants. 



Redifferentiation and regeneration, as here defined, are probably not 

 very different physiologically. Reconstitution in stem pieces of the hy- 

 droids Tiihularia and Corymorpha is wholly redifferentiation; there is no 

 replacement of parts removed, but transformation of the piece. In most 

 planarians both regeneration and redifferentiation occur in reconstitution, 

 and the proportion of each differs in pieces from different body-levels of 

 the same individual and can also be altered experimentally. In general, 

 redifferentiation occurs to a greater extent in the less, regeneration in the 

 more, stably differentiated forms, even within the same phylum or class. 

 In Dugcsia { = Euplanaria), for example, considerable redifferentiation 

 occurs; in Procotyla, another triclad of different family, almost none. 

 Reconstitution in later developmental stages of arthropods and verte- 

 brates is limited to regeneration of certain parts, such as appendages and 

 various tissues. 



The experimental reconstitutions resulting from isolation of parts by 

 section provide material for experimental analysis which cannot be ob- 

 tained in other ways. In various animals they can be initiated at will; and 

 the region of the body, size, and form of the isolated part can be varied 

 and controlled within wide limits, the effects of nutritive condition, physi- 

 ological age, and other factors can be investigated, and much concerning 

 physiological dominance can be learned. 



PLANT RECONSTITUTIONS 



A few examples of reconstitution in plants are briefly described. Re- 

 constitution in certain algae shows graded differences along an axis which 

 are very similar to those in hydroids. The thallus of the dXgd, Acetabular ia 

 mediterranea consists of a long stem from which whorls of hairs develop, 

 an umbrella-like apical region, and rhizoids at the basal end, all formed 

 from a single cell with the nucleus in the rhizoid region. Nucleated pieces 

 reconstitute completely, and even nonnucleated pieces of the stem are 



