SIZE OF CELLS 9 



deeply and irregularly lobed, are found in some of the white blood cor- 

 puscles and in giant cells. 



The size of cells ranges from that of the yolks of birds' eggs which are 

 single cells, at least shortly before being laid down to microscopic struc- 

 tures four thousandths of a millimeter in diameter. The thousandth of a 

 millimeter is the unit employed in microscopic measurements. It is called 

 a micron, and its symbol is the Greek letter p. The small cells referred to 

 are therefore four microns (4 /i) in diameter. The size of any structure 

 in a section of human tissue may be roughly estimated by comparing its 

 dimensions with the diameter of a red blood corpuscle found in the same 

 section. These red corpuscles are quite uniformly 7.5 /x in diameter. 



CYTOMORPHOSIS. 



Cytomorphosis is a comprehensive term for the structural modifica- 

 tions which cells, or successive generations of cells, undergo from their 

 origin to their final dissolution. 1 In the course of their transformation, 

 cells divide repeatedly, but the new cells begin development where the 

 parent cells left off. Cell division, therefore, is an unimportant incident 

 in cytomorphosis. 



Cytomorphosis is a continuous advance in which four successive stages 

 are recognized first, the stage in which the cells are undifferentiated ; 

 s'econd, the stage of specialization or differentiation; third, the stage of 

 degeneration; and fourth, the stage in which the cells die and are removed. 

 These may be considered in turn. 



Undifferentiated cells, as can be seen in sections of young embryos, 

 are characterized by large nuclei and little protoplasm. They multiply 

 rapidly, but the rate of division declines with the gradual increase of the 

 protoplasm and the consequent functional differentiation of the cell. In 

 the adult, relatively undifferentiated cells are found in many situations, 

 as, for example, in the deepest layer of the epidermis. As the cells at the 

 surface die and are cast off, new ones come up from below to take their 

 places. But since the basal cells can produce only epidermal cells, they 

 are themselves partly differentiated. From this point of view the fertil- 

 ized ovum, which can produce all kinds of cells, must be regarded, in spite 

 of its size and great mass of yolk-laden protoplasm, as the least differenti- 

 ated cell. 



Differentiated cells may preserve a round or cuboidal form, but usually 

 they are elongated, flattened, or stellate. The cytoplasm usually con- 

 tains coarse granules, fibrils, masses of secretion or other special forma- 



1 The term cytomorphosis was introduced by C. S. Minot in 1901 in a lecture entitled "The 

 Embryological Basis of Pathology" (Science, ipor, vol. 13, p. 494). Cytomorphosis is further 

 discussed by Professor Minot in "The Problem of Age, Growth, and Death," published by G. P. 

 Putnam's Sons, 1908. 



