48 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Many annelid species regenerate no more than a certain number of seg- 

 ments anteriorly, even though more than that number have been re- 

 moved. This number is characteristic for the species and may be re- 

 garded as representing a more or less specialized head region.'^ Recon- 

 stitution by redifferentiation of some of the old segments posterior to the 

 regenerated anterior end into more anterior segments replacing those 

 which were removed above the number regenerated is known to occur in 

 some species; but for others no data are at hand, though such reorganiza- 

 tion of old segments is probably of very general occurrence. Here, then, 

 as in planarians, reconstitution — at least in certain species — is in part re- 

 generation, in part redifferentiation or reorganization of old parts. 

 In some annehds, however, number of segments regenerated anteriorly is 

 apparently not definitely limited but is more directly dependent on the 

 number removed. In these cases there is probably less or no reorganiza- 

 tion of old segments. Whether a difference in scale of organization in rela- 

 tion to body-level occurs in annelids does not appear from the data avail- 

 able, but an inhibition of head regeneration in relation to body-level and 

 length of piece, much as in planarians, appears in Lumbriculus (Hyman, 

 1916a). As regards existence of a ventrodorsal or a transverse differential, 

 it is found that outgrowth of new tissue from a cut end is, in general, more 

 rapid in the median ventral region than elsewhere. 



Relations between the different axes in certain of the branching 

 bryozoan species resemble, more or less, those in the hydroids. When tips 

 are removed from several branches at the same time, reconstitution begins 

 first on the chief branch, also the ability to regenerate decreases basip- 

 etally in each axis (Otto, 1921). 



In most other animal phyla the capacity for reconstitution is usually 

 more narrowly Hmited. Asteroid starfishes regenerate arms, and a few 

 species are able to regenerate disk and other arms from a single arm with- 

 out any portion of the original disk. Each arm develops like a new polar 

 axis from the tip basipetally, and outgrowth of new tissue is more rapid 

 in the median ventral region than elsewhere; that is, a longitudinal and 

 ventrodorsal gradient are indicated in development of the arms. Rate of 

 regeneration of larval legs of the insect Cloe diptera decreases from anterior 

 to posterior levels (von Ubisch, 191 5). Attention directed to this point 

 would perhaps show similar differences in other arthropods. Some of the 

 ascidians are capable of extensive reconstitution. Pieces of Clavelina from 



■s Morgan, 1897; Hyman, 1916a; Gross and Huxley, 1935; Berrill, 1936". b- Sec also Hyman, 

 1940, Amer. Nat., 74, No. 755. 



