554 ROBERT D. ALLEN 



is held in a glass capillary, the cytoplasmic streaming organization of the 

 cell often remains intact; the fountain streaming pattern continues with 

 the capillary wall serving to replace the destroyed plasmalemma. When 

 the fountain-streaming organization of the cytoplasm breaks down, it 

 characteristically is replaced by one or more loops of streaming cytoplasmic 

 material. In each streaming loop, cytoplasmic material moving toward the 

 bend of the loop corresponds in origin and structure to the endoplasm of 

 the intact cell ; similarly, the material moving away from the bend corre- 

 sponds to ectoplasm. The bend is thus a two-dimensional analogue of the 

 fountain zone, which must have dissociated roughly into radial sections in 

 order to form loops. There is a marked consistency difference between 

 cytoplasm on the two arms of the loop, as can be deduced from the velocity 



Fig. 3. A velocity profile across a loop of streaming cytoplasm dissociated from 

 Chaos clians. Note the difference in velocities (greater in the cytoplasm moving 

 toward the head) and in the shape of the profile. Data of Allen, Cooledge and 

 Hall [5]. 



profiles in Fig. 3. Therefore, the consistency change occurs at the bend. 

 Cytoplasm moving toward the bend slows down, shortens (axially), thickens 

 (radially), and gives up syncretic fluid which can be visualized as vacuole 

 formation in the presence of traces of calcium ions. The observations 

 listed so far are strongly suggestive of an active contraction at the bend of 

 the loop. There is one point of evidence which appears to be conclusive : 

 when cytoplasmic loops stream sporadically, a shortening can be seen to 

 occur a brief moment before displacement of the endoplasmic and ecto- 

 plasmic arms of the loop toward and away from the bend. Thus the 

 temporal sequence of mechanical events provides a seemingly unequivocal 

 indication of an active contraction at the bend of the loops, and therefore 

 probably in the fountain zone of the intact cell as well. 



Some of the behavioural aspects of amoeboid movement which led to 

 doubts concerning the correctness of the tail contraction theory appear to 



