300 



FIELDS AND GRADIENTS 



Fig. 142 



a perisarc which acts as a handicap to all regions within it, and 

 permits of regeneration only at the ends. In Corymorpha, however, 

 the perisarc is absent over most of the stem-length in large specimens. 

 As a result, multipolar forms often arise, especially from short 

 fragments^ (fig. 142). 



A very striking example of the multiple production of dominant 

 regions is seen in the sea-anemone Harenactis, When portions of 

 the body are isolated, they roll up 

 to produce hollow structures like 

 a tyre, the original distal and 

 proximal cut surfaces meeting and 

 growing together. It is then found 

 that regeneration is initiated at a 

 number of places along the line 

 of suture. Regeneration is found 

 especially at places where, owing 

 to irregularities of the cut surfaces, 

 union has not been smooth. At 

 each of these spots, conditions are .tta^anelf^^SS'Te;" 



favourable for new growth, and diagram of Harenactis to show {a and 



therefore the establishment of new ^) sections isolated for regeneration. 



, These roll in to torm hollow tubes, as 



apical organs ; and the various shown (centre) in section : the distal 



regions of new growth are isolated and proximal cut surfaces unite in a 



r 1 .1 1 ^1 • suture, here shown centrally. Right, 



from each other by other regions regeneration of apical regions (whole 



in which smooth union, leaving no or partial tentacle groups) from the 



free cut surface, has taken place.'^ suture. When the suture is irregular, 



' . ^ with considerable prohteration, a 



As Will be seen m the next chap- number of apical regions can arise 



ter, these phenomena of double (above) ; when the union is smooth, 



, . 1 . . . . one regenerate dominates and inhibits 



or multiple organisations arising ^^^ development of others (below). 



from a single portion of tissue are (After Child, Physiological Foiinda- 



of great interest in the interpreta- tions of Behavior, New York, 1924; 



° . ^ modined.) 



tion of various facts in ontogeny. 



Before passing to our next section, the views of Goetsch^ should 

 be mentioned. He finds that regeneration is frequently accom- 

 panied by a polarised migration of cells. In some cases, certain 

 types of cells have the tendency to migrate apicalwards, other types 



Child, 1926. 

 Goetsch, 1929. 



2 Child, 1924, p. 119. 



