EXTENSIONS 375 



place. The cytoplasm would have at any moment a store of these 

 substances, exhaustible if the nucleus is removed. The nucleus has 

 therefore been regarded as the source of the regeneration of enzyme 

 systems present and acting in the cytoplasm (Mazia, 1952). 



Stentors emphasize the importance of this trophic function of 

 the nucleus. The micronuclei are significant only in genetic recom- 

 bination during conjugation. It is the macronucleus which supports 

 the Kfe of the cell, though this involves the expression of specific 

 genetic determinants derived from its progenitor, the micro- 

 nucleus, as was demonstrated for Paramecium by Sonneborn (1947). 

 In metazoa the metabolic function of the nucleus is cryptic and 

 not obvious, or is revealed only by special demonstrations as in 

 neurone regeneration or the somatic deletion studies of Demerec. 

 But in protozoa, as in studies of microbial genetics and the new 

 work on Neurospora^ the trophic role of the nucleus is apparent. 



In stentors, the macronucleus is clearly necessary for both 

 digestion and synthesis which leads to growth. Therefore it should 

 make possible the formation of enzymes and may be a source of 

 RNA, via nucleolar extrusion, for protein synthesis. This nucleus 

 also probably sustains respiration, for though energy metabolism 

 long continues in enucleates it gradually diminishes. Several 

 instances have been cited which show that the quantitative relation- 

 ship between nucleus and cytoplasm is important for these 

 physiological or biochemical processes; and it is in the relatively 

 simple alterability of the nucleo-cytoplasmic ratio that stentors 

 should prove most fruitful in studies of cell physiology. 



Some portion of the macronucleus is essential for cytodifferen- 

 tiation in oral regeneration. Presumably this support is either a 

 synthesis or a mobilization of structural proteins; but in spite of 

 the sameness of stentor organelles this action is fraught with 

 specificities, for the nucleus of one species of Stentor can rarely be 

 exchanged for another to yield an effective nucleo-cytoplasmic 

 combination. And where and how the building blocks are put 

 together in formed organelles is very probably the work of the 

 ectoplasm and its pre-formed structure. 



This view was previsioned by Prowazek in the course of his 

 investigations on Stentor and has been enlarged upon by others up 

 to the present day (Tartar, 1941b; Sonneborn, 195 1; Ephrussi, 

 1953 ; Weisz, 1954; Danielli, 1958). Noting that the nucleus is not 



