Senile Changes in Leaves of Certain Plants 



287 



senile changes in the structure of animal cells 

 All the preceding changes must of course be merely magnified out- 

 growths of changes occurring in the cell. Considering the investigations 

 made as to the effect of senility on the cell itself, it may be seen that very 

 httle has been done toward an actual understanding of this problem. 



Changes evident from the beginning 



Decrease in size of nucleus. — Hodge (1895) found the nuclei of the 

 spinal gangha to be smaller in the senile than in the young condition. 

 Hertwig (1904 and 1906), however, regards the extreme size of the nucleus 

 characteristic of certain protozoan stages and of the animal ovum as 

 evidence of dormancy rather than of rejuvenescence. Minot gives the 

 interesting evidence that while investigators of the embryonic develop- 

 ment of animal eggs make no statement regarding the fact, their drawings 

 invariably show a rapid decrease in the size of the nucleus. 



Increase of cijto'plasm in amount and in diferentiation. — That cyto- 

 plasm increases in amount and in differentiation was first emphasized by 

 Minot (1891), who considers the fact of prime unportance. Eycleshjoner 

 (1904) gives the following data regarding the proportion of cytoplasm to 

 nucleus at different ages of Necturus muscle: 



Length of body 



8 mm. 



17 mm. 



26 mm. 



Adult, 230 mm. 



Proportion of cyto- 

 plasm to nucleus... . 



2,737-1 



4,318-1 



8,473-1 



23,379-1 



The great change in the structure of the cytoplasm by which the highly 

 speciaKzed cells are developed from the undifferentiated cells or the morula 

 stage, is evident. 



Changes appearing later 



Nucleus irregular in form, and staining like cytoplasm. — Irregularity 

 in form of the senile nucleus, and its staining hke cytoplasm, was observed 

 by Hodge (1895) in the spinal gangha of man. 



Local degenerations in cytoplasm. — - Cytoplasm with many local degenera- 

 tions, indicating that portions were undergoing various forms of degenera- 

 tion, is described for ganghon cells by Hodge (1895). Various forms 



