142 MORPHOLOGIC VARIATION 



Throughout the preceding pages I have tried wherever possible 

 to call attention to the parallelism in the phenomena of growth and 

 the accompanying cell changes existing between cultures of bacteria 

 and multicellular organisms. While many biologists have studied 

 the cell changes of plants and animals which occur during growth, 

 senescence, and death, their significance has been especially empha- 

 sized by Minot, who named the process "Cytomorphosis." 



In animals this cytomorphosis is characterized, according to 

 Minot, especially by variations in the ratio of size of the nucleus 

 to volume of protoplasm. This ratio is high in the embryonic 

 cells during the period of most active growth, and becomes pro- 

 gressively lower with increasing age of the animal, the cells be- 

 coming increasingly larger without any corresponding increase 

 in size of the nucleus, although, as he points out, these differen- 

 tiated cells are also characterized by the occurrence of various 

 structures within the protoplasm; the increase in size of the cell 

 being to a certain extent at least due to the accumulation of these 

 granules, vacuoles, and similar structures. Senescence is but a con- 

 tinuation of the same process, the nucleus continuing to shrink. 

 The protoplasm may either atrophy or hypertrophy, according to 

 the nature of the cell, and frequently also still further types of 

 granules or other structures may appear. 



According to Minot, this process is due to intrinsic causes. 

 Once a cell has been started on the growth cycle, it must neces- 

 sarily pass through the remaining phases of this cycle. Growth, 

 senescence, and death are but outward manifestations of this cyto- 

 morphosis. As he states it, "the increase of protoplasm together 

 with its differentiation is to be regarded as the explanation (or should 

 we say cause) of senescence." Further, the process is to a degree 

 irreversible; "after a cell has progressed and is differentiated a cer- 

 tain distance, its fate is by so much determined." This he calls 

 the "law of genetic restriction." 



In the light of later knowledge, however, these last statements 

 can hardly be accepted. The present day tendency is rather to con- 

 sider the changes which occur in the cells of a growing plant or 

 animal not so much a cause of growth as a result. They are mor- 

 phologic manifestations of physiological changes which are occurring 



