290 FIELDS AND GRADIENTS 



ment of Filigrana and Salmacina. The young forms of these worms 

 have three thoracic segments, and the budding zone at the hinder 

 end adds a number of segments of abdominal type behind them. In 

 subsequent development, the number of abdominal segments is 

 increased as a result of the activity of the budding zone, but such a 

 method is of course out of the question in the case of the thoracic 

 segments. These increase their number to ten by conversion of the 

 most anterior abdominal segments.^ 



(v) This fourth rule is really a special case of a more general fifth 

 rule, which is that, within a given field, the diflFerentiation of all 

 regions, other than an apical region, is dependent on influences 

 which proceed from more apical levels. For instance, a piece of a 

 Planarian can regenerate a tail posteriorly even if it fails to re- 

 generate a head. Similarly, whereas a piece of a Planarian from the 

 post-pharyngeal region will not form a new pharynx unless a head 

 is regenerated at its anterior end, a piece from the prepharyngeal 

 region is capable of producing a pharynx even in the absence of a 

 head.^ 



Corymorpha also provides a good example of this. In this 

 hydroid, grafts of a portion of the stem of one polyp inserted later- 

 ally in the stems of other polyps will in a certain proportion of cases 

 act as organisers and induce the outgrowth of a new hydranth. It 

 was found that grafts from the apical region inserted at basal levels 

 induced hydranths in nearly 85 per cent, of cases, while grafts from 

 basal levels inserted at the same level in another stem were only 

 effective in 45 per cent, of cases; in addition, the hydranths pro- 

 duced by basal grafts grew more slowly and arrived at a smaller 

 size.^ The capacity to organise does not reside in any specific tissue 

 but is a physiological condition, the efficacy of which varies 

 quantitatively down a gradient (fig. 138). 



In addition, it should be noted that general stimulation such as 

 that produced by an incision will induce the formation of new 

 hydranths in Corymorpha. Here the influence of the substrate on 

 the result emerges clearly : for whereas at apical levels of the stem 

 a single incision will usually induce a hydranth, at basal levels this 

 is ineflfective, and lacerated incisions are required for induction. 



1 Malaquin, 1919. ^ Child, 1915 A, p. 102. 



3 Child, 1929 B. 



