MORPHOGENESIS 801 



that injuries are apt to be repaired during division. In the types in which 

 dedifferentiation, preparatory to division, is less extensive, there appears 

 to be a greater independence between regeneration and division. Whether 

 or not the imminence of division reduces the possibihty of independent 

 physiological reorganization has yet to be determined. 



Polarity Changes and Protoplasmic Streaming 



The development of temporary heteromorphic individuals in Bursaria 

 during physiological regeneration or in regenerating halves was first 

 described by Lund (1917). The suppression of secondary axes and the 

 reversal of polarity in the weaker member of the heteromorph occurred 

 as a consequence of the dedifferentiation and reorganization of that re- 

 gion. In some instances the secondary axes were so feebly developed that 

 a local reversed ciliary action gave the only clue to their existence. 



The operations on Mastigina hyale by Becker (1928) have a more 

 direct bearing on the mechanisms of polar organization. In normally 

 creeping animals, the nucleus occupies a vacuolar space at the extreme 

 anterior tip, and the endoplasm shows a type of fountain streaming, 

 toward the anterior end in the center and posteriorly at the cell periphery. 

 When individuals are divided into anterior and posterior halves, only 

 the anterior half continues to move as before; motility and streaming 

 are upset in the non-nucleated half, pseudopod formation is interrupted, 

 and death soon occurs. The importance of the anterior extremity in the 

 determination of polarity is demonstrated by the fact that decapitated 

 cells behave just as posterior halves. If the anterior tip, together with the 

 affixed nucleus, is pulled posteriorly with the surface gel, streaming and 

 locomotion cease momentarily and then resume with reference to the 

 new position of the shifted polar cap. But if only the nucleus is dis- 

 lodged from its vacuole and expressed into the flowing endoplasm 

 without severe injury to neighboring parts, the original polarity is un- 

 disturbed, which suggests that the nucleus alone is not responsible for 

 the axiate pattern. Becker assumed that the resistance of the anterior 

 peripheral layer to internal pressure is reduced by the physical presence 

 of the "foreign body" in the gel. The kinetics of movement in Mastigina 

 are interpreted in the following way: "it is the entire anterior tip of 

 permanently gelled protoplasm which prevents gelation of the 'endo- 

 plasm' immediately behind it, and which by imperfect continuity with 



