MORPHOGENESIS 773 



as polarity and symmetry, which, according to Child (1920), appear to 

 be largely independent of specific differences in the protoplasmic con- 

 stitution. Should not the Protozoa, either as cells or organisms, present 

 very suitable material for studying the potentialities of the protoplasmic 

 constitution in a somewhat less complex setting than that which charac- 

 terizes the higher invertebrates? Furthermore, does the disadvantage of 

 the operative techniques involved with Protozoa offset the advantage of 

 being able to disregard some of the physiological and mechanical rela- 

 tionships between the cells of compact tissues? The answers to these 

 and other questions relative to the significance of Protozoa in connection 

 with problems in regeneration, or their merits as material for investi- 

 gating these problems, are to be attempted in this section. 



It is the purpose of this section to organize under special headings the 

 unassembled work having to do with regeneration phenomena in 

 Protozoa. Microdissection or micrurgical studies which have as an ob- 

 jective the elucidation of the physical properties of protoplasm, the 

 functions of fibrillae, membranes, and so forth, are to be treated only 

 insofar as they have a bearing upon the topics under consideration. 



Physiological Regeneration 



One aspect of regeneration pertains to those functions by which cells 

 or organisms are able to maintain a certain structural and functional in- 

 tegrity, in spite of wear-and-tear processes of the normal life cycle. The 

 maintenance functions are commonly referred to as physiological re- 

 generation and they appear to have much in common with reparative 

 regeneration. 



As applied to Protozoa, particularly the Infusoria, the term has an 

 added significance because it should be taken to include not only the 

 gradual and continuous energy changes within the protoplasm, but also 

 the grosser reorganizational changes encountered during periods of di- 

 vision, conjugation, endomixis, and encystment. In ciliates and flagel- 

 lates generally, the structures observed to be affected include particularly 

 the nuclei, the external motor organellae, and their associated fibrils. 



It is beyond the scope of this section to consider those repair phe- 

 nomena which follow the normal vegetative or cyclical variations in 

 Protozoa. Of especial interest in this connection, however, is the "spon- 

 taneous" dedifferentiation and redifferentiation described for Bursaria 



