REGENERATION 



163 



\^ 



Fig. 57. Reorganisation in the hydroid polyp Tubularia. A very small 

 part produces only (a): the apical part of a polyp; if more material 

 is present, the more basal parts are formed accordingly (b-d). 



After Child. 



that occur after the transection of hydroid polyps, though here 

 as a rule we find no regeneration, but only a "reorganisation" 

 of the old tissues, which are subjected to a morphological de- 

 differentiation, followed by new differentiation. If a stem 

 fragment of such a polyp is isolated, it will change into a new 

 hydranth provided the size of the fragment is sufficient. Smaller 

 parts produce only the apical parts of a polyp, but these apical 

 parts are of normal size^). Evidently, the process of re- 

 organisation begins with the formation of the apical parts, 

 and proceeds from there in the direction of the base, until 

 all the available material has been used (Fig. 57). 



The size of the apical parts formed depends more or less 

 upon the external circumstances. In Planaria, for instance, only 

 a small head is produced by a regeneration-bud at low 

 temperatures, or under the influence of anesthetics. High 

 temperatures, on the other hand, result in the production of 

 very big heads (Child, 1915), (Fig. 58). In the hydroid 

 Tubularia, the size of the hydranths produced depends on the 



1) In many cases, apical parts arise at both ends of the stem fragment. 

 We shall not enter into this complication. 



