RECONSTITUTIONAL PATTERNS IN EXPERIMENT 369 



gions, of various annelids.'^ As in planarians, level of origin of piece, gra- 

 dient differential in it, and intensity of activation at the cut ends may be 

 the factors determining at least some of these bipolar forms rather than 

 specilic differences in potency for head or posterior development at differ- 

 ent levels. 



Pieces of the ascidian Clavellina from the "esophageal" region may re- 

 constitute complete individuals or become bipolar partial forms by de- 

 velopment of siphons, branchial chamber, and other distal parts at both 

 ends (Brien, 1934). Stolons attached to the body develop new individuals 

 at their distal ends; stolon pieces isolated by section reconstitute individ- 

 uals from their proximal ends, probably because activation of cells at this 

 end by section and isolation is sufficiently intense to establish dominance 

 and a gradient which inhibits the less rapid bud development at the distal 

 end. The bipolarity of Clavellina pieces is certainly not due to restriction of 

 potency, for experiment has shown that proximal, as well as distal parts, 

 can develop from the proximal ends of pieces like those that become 

 bipolar, and there is evidently no limitation of potency in the stolon. 

 Brien's interpretation of his experimental data is in terms of gradient 

 relations. 



The relative roles of reorganization of old parts and regeneration of 

 new tissue vary in different species and groups among the lower inverte- 

 brates; in some planarians they differ in definite ways with body-level of 

 origin of piece and with experimental conditions. In the higher inverte- 

 brates and in vertebrates, reconstitution in mature individuals is chiefly 

 or wholly by regeneration and is, in general, limited to regeneration of 

 subordinate parts, such as appendages, or in higher vertebrates almost en- 

 tirely to regeneration of a particular tissue by cells of that tissue. 



Regeneration of legs of arthropods and amphibians evidently involves 

 origin of a new longitudinal axiate pattern. The regenerate is at first bud- 

 like in appearance; but, as regards gradient pattern, little is known. In 

 the regenerating legs of certain insect nymphs (Agrionidae) the tarsal 

 claws usually become distinguishable as definite, localized outgrowths in 

 very early stages, long before muscle attachments and articulations ap- 

 pear; that is, the first visible evidence of structural differentiation is at 

 the tip of the regenerate. Development of muscles, tendons, and articula- 

 tions, however, generally progresses from the proximal region distally; 

 and when regeneration begins in later nymph stages muscles and articula- 

 tions are often absent from the distal region, or the whole leg may have 



'Morgan, 1902; Korschelt, 1904, 1927-31; Gates, 1927; von Haffncr, 1931. 



