SENESCENCE IN HIGHER ANIMALS AND MAN 



285 



and cell bodies indicate that there is comparatively little change 

 in proportion during the process of differentiation. Of course 

 such measurements are not exact, and, besides, the measurements of 

 the cytoplasm do not include the dendrites and the nerve fiber 

 arising from the cell: if the volume of these were added to the 

 cytoplasmic volume of the cell the total would undoubtedly show 



113 



114 



FIGS. 113, 114. Cells from the nervous system of the chick embryo: Fig. 113, 

 embryonic cells from neural tube at 31 hours; Fig. 114, differentiated motor 

 cells from spinal cord at n days, drawn to the same scale. From embryological 

 preparations of the University of Chicago. 



an increase in cytoplasmic volume during differentiation. But 

 how can the dendrites and the nerve fiber contribute to decrease the 

 rate of metabolism in the cell body, since they are merely slender 

 outgrowths from it ? The cell body has unquestionably undergone 

 senescence during differentiation, but without any very great 

 change in the nucleo-cytoplasmic relation. A marked proportional 



