PROTOZOA AS CELLS 69 



wispy material represents an evolution of chromatin bodies by 

 condensation of nucleoprotein which aggregates then to form 

 hollow granules and ultimately the patternless normal chromatin 

 granules. However, no similar process was noted in normal young 

 reproducing cells. Interestingly enough, Rudzinska (1956, and 

 references cited there) found that overfeeding of young animals 

 greatly reduces their life span and leads to hemixis and other 

 changes associated with age. Their macronuclei show large 

 chromatin masses with the same ordered structure as those of 

 old cells. 



Similar structural organization in the chromatin of developing 

 spermatid nuclei of invertebrates has been demonstrated by a 

 number of investigators (Yasuzumi and Ishida, 1957; Fawcett, 

 1958). Ultimately in these cases the crystalline pattern disappears 

 as the nuclear contents become so dense as to conceal any internal 

 structure. Presumably, changes in morphology from moderately 

 dense and homogeneous to lamellar to crystalline to highly 

 condensed coincide with alterations in the nuclear proteins and 

 their bonding with DNA. At any rate, no degenerative change 

 in the chromatin can be involved. 



Morphological evidence of the movement of high-molecular- 

 weight substances from nucleus to cytoplasm has been sought by 

 many electron microscopists; several examples of pictures that 

 could be interpreted as illustrating such a passage — through 

 pores or via membranes — have been cited above. Additional 

 examples susceptible to such interpretation are available in 

 literature on ultrastructure of metazoan cells (Bernhard, 1959). 

 But in no case are unequivocal sequences of stages shown. The 

 paucity of evidence is rather surprising in view of the frequency 

 of observations on nuclear ejections in the light-microscope 

 literature. Ehret and Powers (1955) present phase micrographs of 

 unfixed squash preparations of Paramecium bitrsaria indicating that 

 intact young nucleoli are extruded from developing macronuclear 

 anlagen following conjugation, but they could not illustrate this 

 process in electron micrographs. However, they noted a general 

 similarity in appearance between young nucleoli and small dense 

 mitochondria in the perinuclear cytoplasm. 



For Amoeba, an abundance of data is available from biochemical 

 and tracer studies (see Brachet, 1958, 1959; Plaut, 1959). For 



