14 PSYCHOBIOLOGY 



that " both nucleus and cytoplasm arise through the division of the cor- 

 responding elements of a preexisting cell ", 2 or, more succinctly, " Omnis 

 cellula e cellula ". 8 



Protoplasm is not a single definite substance, but varies greatly from 

 cell to cell. The protoplasm of a nerve cell, for example, is different 

 from that of a liver cell. The nucleus of any cell, moreover, differs physi- 

 cally and chemically from the remaining protoplasm of the cell. The 

 chemical structure of protoplasm is in any case exceedingly complex, the 

 chief constituents in point of quantity being carbon, oxygen, nitrogen, 

 hydrogen, sulfur, fosforus, chlorin, sodium, potassium, calcium, mag- 

 nesium, and iron. Certain organisms include still other elements in their 

 protoplasm. 



Fig. 2. Ciliated cells; bacilli of typhoid fever. (Sedgwick and Wilson, General 

 Biology.) An example of a unicellular plant. 



Some cells are approximately spherical in shape ; -among these are cer- 

 tain eggs, and certain unicellular plants. In most cases, however, the form 

 is modified by growth in special directions, or by pressure of surrounding 

 cells or other structure. In size, cells are usually microscopic, the diam- 

 eter of many of the human cells being as low as four one-thousandths 

 (.004) of a millimeter (usually written 4/u-; read four micro-millimeters, 

 or four mikrons). 4 Human red blood corpuscles are quite uniformly 



2 Schultze, Arch. f. Anat. u. Physiol., i86i, S. II. 



"Virchow, Arch. j. Pathol. Anat., 1855, VIII, S. 27. 



4 There is some confusion in regard to the term ' micro-millimeter '. Certain 

 authors (mainly physiologists) use it to designate the one-thousandth part of a milli- 

 meter; other authors (mainly physicists) employ it to signify the one-millionth part 

 of a millimeter. Conventionally, the prefix ' micro-', when it is the solitary prefix to 

 the name of a unit of measurement, means the millionth part of that unit. One 

 micro-volt, for example, is the one-millionth part of a volt. The use of ' micro-' before 

 another prefix, as in ' micro-millimeter ' is unfortunately not standardized. 



The Greek letter corresponding to the initial of a standard of measurement always 

 represents the one-thousandth part of the smallest common unit of that standard. Thus, 

 C- (mu) indicates the thousandth part of a millimeter, and ffi (mu-mu) represents the 

 millionth part of a millimeter. Correspondingly, a (sigma) indicates the thousandth 

 part of a second, and y {gamma) if used would represent the thousandth part of a 

 milligram. 



