210 GENERAL EMBRYOLOGY 



tions in the structure and composition of the cell. Altogether 

 these modifications are known as senescence. A frequent 

 characteristic of a senescent cell, in both Protozoa and Metazoa, 

 is the relatively large proportion of cytoplasm as compared 

 with nuclear substance. It is further assumed that syngamy 

 and the consequent admixture of nuclear and cytoplasmic 

 materials of two individuals, perhaps representing different 

 races, causes the restoration of the senescent protoplasm to a 

 condition of vigor, in a word, brings about rejuvenation. It 

 would follow from this, that protoplasmic activity is cyclic, 

 and that periods of senescence would be followed by death un- 

 less fertilization, or an equivalent process, should occur. 



Precisely what is involved in the process of rejuvenation 

 cannot be stated definitely. Richard Hertwig suggests that 

 senescence is due chiefly to changed nuclear-cytoplasmic rela- 

 tions resulting from repeated cell-fissions, and that in rejuve- 

 nation there is essentially a restoration of the normal nuclear- 

 cytoplasmic ratio, as well as a certain chemical and physical 

 reorganization of the protoplasm through the combination of 

 materials from two more or less unlike individuals. Loeb and 

 others, as we have seen, also regard the rapid synthesis of 

 nuclein as the most important consequence of fertilization. 

 Still others (Minot, Bernstein) believe that rejuvenation is not 

 only a nuclear-cytoplasmic phenomenon involving or resulting 

 from an increase in the relative amount of nuclear substance, 

 but that it further includes an increase in the property of growth, 

 i.e., the formation of new protoplasm, both nuclear and cyto- 

 plasmic, out of non-living substance. 



The real evidence for the cyclic character of the life processes 

 of the Protozoa is chiefly that of Maupas and Calkins, who 

 showed that in Paramoecium and some other Ciliates, when 

 conjugation is prevented, there occur, under laboratory condi- 

 tions, periods of depression in vital activity, accompanied by 

 changes in structure, i.e., periods of senescence. This depres- 

 sion leads finally to death unless conjugation is permitted, or 

 unless the organisms are subjected to some form of stimulus. 

 If stimulated by chemical or physical means, or naturally 



