166 



REGENERATION 



fragment. This develops into a new pharynx sheath, and breaks 

 through to the exterior, forming a new mouth. The pharynx 

 itself is formed by the regeneration blastema and grows into 

 the sheath from the rostral side (Van Asperen, 1946). 



Morphallaxis is very clear in Sahella. In this worm, the 

 foremost 5 to 11 segments, the so-called "thoracic" segments, 

 are distinguished by a number of characters from the more 

 caudal "abdominal" segments. If the animal is transected in 

 the abdominal zone, the posterior part regenerates a head 

 consisting of three segments. This, however, is followed by 

 changes in a number of the foremost abdominal segments, 

 which thereby acquire thoracic characters (Berrill, 1931). This 

 reorganisation proceeds in a rostro-caudal direction (Fig. 59). 



Fig. 60. Organising activity of a transplanted head in Planaria. 

 (a) a laterally implanted graft has induced a lateral outgrowth 

 and a secondary pharynx, (b-c) a subterminal graft has induced 

 an outgrowth (directed forwards), and two phar3niges; (d) a ter- 

 minal graft has caused a reversal of polarity in the caudal part of 

 the host, and a secondary pharynx; in this last case the hind part 

 was capable of autonomous movement (dotted outline), i.e. it be- 

 haved as an independent individual. After Santos. 



