284 SENESCENCE AND REJUVENESCENCE 



life than later. Certain organs also undergo characteristic changes 

 in constitution, but the relation between these changes and the age 

 cycle is in most cases not yet clear. 



THE MORPHOLOGICAL CHANGES 



If senescence is merely one aspect of progressive development 

 the morphology of senescence in man and the higher forms is simply, 

 as elsewhere, the morphology of progressive development. The 

 morphological changes in the cells consist in general of the appear- 

 ance of more or less definite structural substances, which dififer in 

 form and character according to the direction of differentiation in 

 particular cells or organs. Morphological differentiation of the 

 cell involves either an accumulation in the cytoplasm of substances 

 different in appearance and constitution from the cytoplasmic sub- 

 stratum of the embryonic cell, or a replacement of the embryonic 

 substratum by such substances. This process of differentiation, 

 or cytomorphosis as Minot prefers to call it, very commonly involves 

 an increase in the volume of the cytoplasmic portions of the cell 

 as compared with the nucleus. In embryonic cells the nucleus is in 

 general, relatively to the cytoplasm, larger than in differentiated 

 cells. Minot has laid particular emphasis on this change in the 

 proportion of nucleus and cytoplasm as a fundamental feature of 

 progressive development and as the determining factor in the 

 decrease in metabolic rate which occurs in senescence. Such a 

 change undoubtedly does occur in at least many cells in the course 

 of differentiation, particularly in the higher animals, but it is by 

 no means universal, as Minot maintains. In certain of the lower 

 animals there is little if any difference between the embryo and 

 the adult in this respect, and the differentiation of plant cells is 

 very generally accompanied by vacuolization rather than by in- 

 crease of cytoplasm. 



Figs. 113 and 114 show embryonic and differentiated cells from 

 the spinal cord of the chick. The cells in Fig. 113 are from the 

 neural tube soon after its formation, and in Fig. 114, drawn to 

 the same scale, nerve cells from the spinal cord after eleven days 

 of incubation, at which time some of the nerve cells have attained 

 practically their full size. Measurements of the volume of nuclei 



