CELLS IN DIVISION 



made sufficiently clear in the past. It should now be evident that 

 there are very considerable differences, which arise mainly because 

 different particles are held together by unlike forces in the two cases; 

 in one, moreover, the structure is built around a number of definite 

 orientating centres, while in the other it is not. It might be argued 

 that although there are differences in detail between the two systems, 

 nevertheless spindles fall within the definition of tactoids given 

 earlier, and the general nature of the forces that govern the patterns of 

 aggregation is the same in both cases. This may be so; but if the tactoid 

 hypothesis has to be whittled down to such vague generalities, it 

 becomes doubtful whether it is worth having. In short, while it is not 

 incorrect to call the spindle a tactoid, providing the definition of 

 tactoids is wide enough, there is little point in doing so. 



GENERAL CYTOPLASMIC CHANGES DURING MITOSIS 



It is probably safe to say that all activities and features of a cell are 

 affected by mitosis. The visible changes are most marked in non- 

 spherical cells, which lose their characteristic shape and become 

 rounded during division. This is seen in columnar epithelia, as in the 

 gut wall of the lower vertebrates (Cohen and Berrill^'^) and to a 

 variable extent in the flattened fibroblastic type of cell in tissue culture, 

 where the angular prolongations of the cell are withdrawn, leaving 

 behind fine filaments which connect one cell with another; at the 

 same time, the cell increases in thickness. These changes proceed 

 slowly during prophase, but when the nuclear membrane suddenly 

 disappears at the end of this period, there is an equally rapid rounding 

 of the whole cell. 



During mitosis some specialized functions both of the external surface 

 and of the cytoplasm are suspended. Cilia are lost from the epithelial 

 cells of the frog oesophagus (Kindred^^^) ; in the hypotrichous ciliates, 

 all the motor organelles are resorbed during fission and two new sets 

 are formed in the daughter individuals. This fact was realized by 

 Stein^'^' in 1859; Calkins^'^ is of the opinion that similar events occur 

 in all the main group of the Protozoa. Cleveland^^^ has described 

 the complex events which occur during cytokinesis in flagellates such 

 as Holomastigotoides where the daughter nuclei separate as the spiral 

 flagellar band to which one nucleus is attached unwinds round the 

 organism (Figure 49). 



The changes at the surface of Acanthamoeba during division are 

 described by Comandon and de Fonbrune^^o in these terms: "ce n'est 

 qu'immediatement avant la mitose qu'elle cesse de se nourrir et de 

 ramper. Alors son contour s'arrondit et se herisse de nombreux, courts, 

 et fins pseudopodes qui sont de moins en moins mobiles'. 



133 



