PROCESS OF REGENERATION 143 



I\'. ANALYSIS OF THE REGENERATIVE PROCESS 



A consideration of the data presented above leads us to the 

 conchision that the formation of an anterior end is a different 

 ])rocess from the formation of a posterior end. The fact that in a 

 set of pieces of Lumbricuhis of equal size, and taken from the 

 same level of the body, any type of anterior outgrowth from a 

 normal head to a normal tail may be obtained, proves that the 

 kind of anterior structure which is to arise is not predetermined 

 in the piece, but is due to dynamic conditions arising within 

 the piece after its isolation from the whole. The previous 

 position of the piece as part of the whole is only an indirect factor 

 in head determination. 



The consideration of a similar set of facts obtained from his 

 experiments with Planaria dorotocephala led Child ('13 b, 

 '14 e) to an analysis of anterior regeneration in this animal in 

 metabolic terms. In the following pages a similar analysis is 

 made for Lumbriculus inconstans, 



1 . The time of head detennination 



It is necessary to know, as the first step towards the under- 

 standing of the process of anterior regeneration, at what time 

 the head is determined in a regenerating piece. This can be 

 readily discovered in the case of organisms which show progres- 

 sive inhibition of head formation along the axis by means of the 

 method devised by Child ('14 e). The piece ac in figure 23 gives 

 rise to normal heads in 90 to 95 per cent of cases (the remaining 

 forming mostly multiple structures). The piece ah with the 

 same anterior level will, however, produce under ordinary con- 

 ditions only 20 to 30 per cent of normal heads, and 70 to 80 per 

 cent of inhibited heads and other types of anterior outgrowths. 

 It is therefore obvious that whether or not the cells shall give 

 rise to a normal head is dependent upon their forming part of 

 the long piece ac; and if they remain a part of ac for a sufficient 

 length of time, then they should, when isolated, give rise to noth- 

 ing but normal heads. To discover the period of time required 

 for the determination of the head as norMal, it is only necessary 



