REGENERATION 3I7 



commonly found that embryos in the most active phases of development 

 have less power of regeneration than at either earlier or later more 

 quiescent periods. Thus in the Amphibia the early gastrula can easily repair 

 defects, and so can the young tadpole, but the neurula or early taU-bud 

 stage has little capacity for regulation. 



{b) Flatworms 



It will be useful to supplement the account of regeneration fields in 

 hydroids by a somewhat shorter discussion of similar phenomena in flat 

 worms, which are also a lowly group of invertebrates but somewhat more 

 highly evolved than the coelenterates (General Reviews: B rousted 

 1954^, b, Wolff 1953). We shall fmd that in the flatworms the active 

 individuation field in the adult is less powerful than that wliich is main- 

 tained by the hydranths in hydroids. As a consequence of this the main 

 role in directing the early course of regeneration is played not by the adult 

 organs which remain in the regenerating piece, but rather by a static field 

 of regeneration potential, which can be compared with the gradient in 

 regeneration rate which was characteristic of the hydroid stem. This 

 determines the character of the blastema which is formed, and that in its 

 turn then brings about the development of the appropriate organs. 



Regeneration has been mainly studied in triclads, of which Planaria is 

 a characteristic genus. The whole group are frequently referred to as 

 planarians. If a planarian is cut in two by a transverse cut it is frequently 

 found that the anterior segment regenerates a tail and the posterior seg- 

 ment a head. In both cases the process starts by the formation of a blastema, 

 which is produced by neoblasts which migrate to the wound surface (see 

 p. 306). Many species of planarians are provided with eyes in the anterior 

 region of the head. One of the first signs of head regeneration is the ap- 

 pearance of such eyes in the blastema. This occurs at an early stage, even 

 if a large part of the anterior of the worm has been removed. Thus it 

 seems that in regeneration of the head the most anterior part is formed 

 first and whatever else is required is, as it were, intercalated between this 

 anterior part of the head and the remaining posterior end of the body. 

 During the later stages of regeneration, however, a good deal of morphal- 

 laxis occurs, that is to say a remodelling of the original posterior part of 

 the body. 



The occurrence of these easily recognised organs, such as eyes, makes it 

 simpler to study the regeneration of the head than that of the tail, and 

 most work has concerned itself with this type of regeneration. The sim- 

 plest experiment consists in placing the transverse cut, which divides the 

 worm in two, at various levels from the anterior to the posterior. We fmd 



