PROTOPLASM OF PROTOZOA 85 



the plasmagel takes place, resulting in the formation of numerous 

 pseudopodia which become attached and undergo active ameboid move- 

 ment. At the same time, because of the flow of the cytoplasm from the 

 equator toward the two poles, the region of the Amoeba at the equator 

 has become narrowed to a thin neck. Presumably a solation of the 

 plasmagel at the equator, together with a pull exerted by the two ac- 

 tively dividing ameboid daughter cells, produces the final separation of 

 the organisms. The temporary polarity becomes immediately lost after the 

 completion of division. 



The physical-chemical factors involving the change in polarity of the 

 protoplasm at the equator and the forces responsible for the flow of the 

 protoplasm from this point are unknown. However, Chalkley thinks 

 the fundamental principles involved in these processes are the same as 

 those described by Mast for ameboid movement, namely sol-gel trans- 

 formation. Further, it has been shown by Chambers (1938) that if the 

 nucleus is moved toward the cell surface, pseudopodia are induced in 

 that region. Becker (1928) has demonstrated that the factor which de- 

 termines the direction of streaming and hence polarity is located in 

 Mastigina, a flagellated amoeba, in the region of the nucleus. If the 

 nucleus is moved posteriorly, streaming ceases and is then resumed to- 

 ward the nucleus. 



It should be pointed out that in the Protozoa nuclear division and 

 cytoplasmic fission may be closely correlated or widely separated in tempo, 

 and that they often exhibit a considerable degree of independence. How- 

 ever, the plane of separation of the nucleus usually determines the plane 

 of cytokinesis, in that they usually take place at or nearly at right angles 

 to each other. In organisms like the Mastigophora, the plane of nu- 

 clear division is parallel to the anterior-posterior axis and coincides 

 with the plane of cytokinesis of the organism. In Ciliata the plane of 

 nuclear division is perpendicular to the anterior-posterior axis and coin- 

 cides with the plane of cytokinesis. 



In some ciliates there appears to be a definite and permanent divi- 

 sion zone laid down early in the life of the organism, which is not 

 disturbed by diverse multilations of the body (Calkins, 1926). Further- 

 more, in Fronton/a this zone differs so markedly from the surrounding 

 cytoplasm that it can be easily seen in the living condition (Popoff, 

 1908). 



