128 LIBBIE H. HYMAN 



Another general feature of oligochaete regeneration is the 

 inability of the extreme anterior and posterior ends to regenerate. 

 This was noted by Bonnet, and by most investigators since. 

 The head, using the term in its strict sense, when isolated, never 

 regenerates a posterior end; it either dies very soon or else gives 

 rise to 'tailless' heads. The head must have a certain minimum 

 number of body segments behind it before posterior regener- 

 ation can occur. This fact, which has long puzzled workers 

 on regeneration, is readily explained on the basis of Child's 

 investigations (a preliminary discussion of these matters 

 appears in the second of the Studies, '11 e). He has pointed 

 out that a large number of data from plants and lower inverte- 

 brates show that anterior regions are physiologically dominant 

 over posterior, and the most anterior region, the apical end in 

 plants, and the head in animals, dominates over and controls 

 all other parts of the organism. In regulation, the first step is 

 the formation of a dominant region, and the development of 

 other parts then follows in correlation with this. For the forma- 

 tion of a dominant part, the cells which are to form it must have 

 a rate of metabolism sufficiently high to enable them to become 

 physiologically isolated and independent of the rest of the piece. 

 If the piece does not contain sufficient material for both domi- 

 nant and subordinate parts, then the latter cannot arise, as in 

 the case under discussion. This production of dominant parts 

 and nothing else from short pieces is a familiar fact in the case of 

 many coelenterates and flatworms, and occurs also in Lum- 

 briculus. Such pieces may also give rise to biaxial dominant 

 structures, if the cells at the posterior end can attain a rate of 

 metabolism sufficiently high to enable them also to become 

 physiologically isolated. Under such conditions, a dominant 

 structure will arise in each of the physiologically isolated regions, 

 namely, at each end of the piece. A piece which does not con- 

 tain enough material for both dominant and subordinate parts, 

 or which consists to begin with of a dominant part only, can 

 therefore do one of two things, — form a tailless structure, or 

 give rise to biaxial dominant parts. 



