38 GENERAL CONSIDERATIONS 



d'etre of all sorts of subsequent peculiarities, but for the phenomenon 

 of increased vitality under consideration, a very simple experiment with 

 Uroleptus mohilh shows that fertilization with amphimixis has little to 

 do with the problem. A conjugating pair is cut across the apex with a 

 scalpel. One individual of the two thus separated is fixed and stained, to 

 show the stage of conjugation when cut, while the other one is cultivated 

 in an isolation culture dish. The experiment is particularly striking when 

 the two wandering pronuclei are cut off with the apical piece ( Fig. 13). 

 The cultivated individual goes through exactly the same processes as 

 though conjugation had been completed in the usual manner, and a 

 normal life cycle results. Amphimixis, however, had been prevented; 

 conjugation apparently had been transformed into endomixis (Calkins, 

 1921). Here, however, the possibility remains that one of the super- 

 numerary micronuclet of the amputated individual may divide and one 

 part may unite with the pronucleus already present. 



The loss and replacement of the old macronucleus, together with 

 that of the old derived structures generally remains as a possible ex- 

 planation. This phenomenon is common to endomixis, conjugation, and 

 conjugation merotomy, and in all cases a renewed vitality results. This 

 does not happen with division, although, as shown above, characteristic 

 changes occur in the macronucleus. With its disintegration and re- 

 sorption in the cytoplasm, a new supply of nucleoproteins and other 

 chemical compounds useful in metabolic activities are distributed in the 

 cell, and these may be potent factors in the new vitality. Added to these 

 is the fact that an entirely new and powerful organ of the cell — the 

 macronucleus — has been supplied from the fundamental organization. 



The majority of the derived structures of the cell have a relatively 

 short life, being resorbed and renewed at division (cirri and other 

 motile organs; others, such as membranelles and oral structures, appar- 

 ently have a longer life. Of all derived structures the macronucleus has 

 the longest life; it divides at cell division and may remain functional 

 through entire life cycles, including many hundreds of divisions. It is 

 probably the chief metabolic agent of the cell, yet it apparently lacks 

 the power of continued life which the micronucleus possesses. It is 

 essentially somatic in character, and, like the soma in metazoa, it ulti- 

 mately wears out; if not replaced, the rest of the cell, including the 

 micronucleus depending on it, dies with it. It is quite possible, indeed 



