REGENERATION 167 



There are a number of experiments that leave no doubt that 

 morphallaxis is due to "organising" activities of the previously- 

 regenerated head. Santos (1929) transplanted parts of the 

 head of one planarian into the caudal region of another. The 

 graft regenerated the missing parts of the head, and induced 

 a pharynx some distance away in the host tissues. The direction 

 of this pharynx agreed with that of the implanted head. In 

 many cases the host tissue even supplied a complete new caudal 

 part, adjusted to the implanted head (Fig. 60). Similar effects 

 were obtained in the case of heteroplastic transplantation 

 between two species of Planaria. 



In hydroids, too, the existence of such organising activities 

 has been demonstrated. Here they are exerted by the apical parts 

 of the polyp. In Hydra, the peristome (i.e. the area around the 

 mouth) of one polyp, implanted laterally into the body of an- 

 other, induces the formation of tentacles and the production 

 of a bud. 



The comparison is obvious between these organising in- 

 fluences of regenerated heads or apical parts and the activity 

 of organisers in embryonic development. Here, again, the 

 situation can be described as an "organisation-field", originating 

 from the regenerate and spreading from there over the adjacent 

 areas of the original fragment; the role of organiser is here 

 played by the regenerate. Under the influence of the field, the 

 cell material undergoes certain changes characteristic of 

 morphallaxis, changes which adapt regenerate and fragment 

 to each other. Therefore, the study of regeneration may deepen 

 our insight into the properties of organisation-fields. 



First of all, the field in certain cases appears to adapt its 

 size to the available material. In a fragment of the ascidian 

 Clavellina, for instance, complete de-differentiation of the 

 tissues into a compact mass of cells may occur. This is then 

 followed by fresh differentiation into a well-proportioned 

 Clavellina of smaller size (Huxley, 1926). Evidently the field 

 in this case harmoniously imposes itself on such material as 

 is present; its size is adjusted to the available quantity of cells. 

 However, that this is not always so, may be seen from the 

 case of hydroid polyps described above, in which fragments 



