24 GENERAL CONSIDERATIONS 



the chromatin does not go into solution in that part of the band known 

 as the "solution plane," but appears there as closely packed granules 

 and then passes to the part called the "reconstruction plane," not as 

 precipitated granules, but as granules which have undergone some 

 physical or possibly chemical change. Turner (1930), working on Eu- 

 plotes patella, cites the absence of granules in the solution plane and re- 

 gards the substance of this plane "as in the state of a colloidal solution." 

 The chromatin reticulum in the center of the nucleus he interprets as in 

 a continuous phase, while the karyolymph is dispersed. After action in 

 the reorganization bands this condition is reversed, the chromatin gran- 

 ules being in the dispersed phase and the karyolymph continuous. Phe- 

 nomena of an exactly similar nature have been observed in Diophrys, 

 Stylonychia, and a host of other forms so that little doubt can be enter- 

 tained as to the probability that this is a highly critical period in the daily 

 life of a ciliated protozoan. 



At the time of division, not only in the macronucleus but throughout 

 the organism there is a wave of general house cleaning. Morphological 

 parts which have been active in the metabolic reactions receive attention 

 and all are built up afresh. This principle was recognized and applied 

 to all parts of the cell by Wallengren in 1900 and again in 1901 when 

 he wrote his illuminating paper in German. 



In the Hypotrichida in particular, in which there is no covering of cilia 

 but instead motile organs called cirri, of a more complex type, Wallen- 

 gren was the first to show, in great detail, that the old cirri, just prior 

 to cell division, are gradually resorbed, while new ones arising from 

 the cortex at an adjacent spot, grow out slowly to take their places. In 

 like manner cilia are also replaced, and thus it comes to pass that, inside 

 and outside, the active organs of a ciliate are composed of new materials 

 derived from the fundamental organization contained in every protozoon. 



It is not only the ciliates that possess this apparent fountain of eternal 

 youth; other groups of Protozoa manifest similar, if not identical phe- 

 nomena. 



With few exceptions, cell division in flagellates is longitudinal, be- 

 ginning as a rule at the anterior or flagellar end, the cleavage plane 

 passing down through the middle of the body. As the process continues, 

 the two daughter cells separate and usually come to lie in one plane, so 

 that final division appears to be transverse. 



