18 



GENERAL ZOOLOGY 



scope is found to consist of fragments of liver tissue in which the 

 typical outlines of liver cells can be seen (Fig. 5). These 

 fragments are also protein in nature. Thus in cells from which 

 all of the soluble protein has been extracted there still remains 

 what may be considered a structural protein upon which the 

 morphology of the cell depends. The structural protein is 

 colloidal, i.e., its demonstration depends upon the fact that it is 

 less soluble than the other protein constituents. 



Cells as a rule are microscopic in size. Cells of macroscopic 

 proportions are the result of special conditions. Thus the 



Fig. 5. — Section of the liver of rabbit frozen in liquid air, dried at — 30°C, 

 extracted for 48 hours with salt solution, 48 hours with distilled water, fixed in 

 formalin and stained. Cell form and protoplasmic structure is still intact and 

 some remains of nuclear content may be seen. X 300. {After R. R. Bensley 

 and N. L. Hoerr.) 



unfertilized egg of the domestic fowl is a cell which is approxi- 

 mately the size of the yolk of the laid egg. (The white of the egg 

 and the hard shell are protective coverings.) In this case the 

 large size of the cell is accounted for by the yolk material stored 

 in the cytoplasm of the egg cell. The yolk-free cytoplasm and 

 the nucleus are of microscopic volume. The large size of eggs 

 generally is due to the presence of yolk material in the cytoplasm. 

 The common unit of linear measurement employed in microscopy 

 is the micrometer or micron, abbreviated as p, (Greek letter n), 

 which is a length of 0.000001 meter. Human red blood cells 

 measure 7.5 to 8.5 /j, in diameter; some leucocytes about 10 /x; a 

 skeletal muscle fiber, which is really a multinucleated cell, may 

 be 1 in. long and 50 /j. in width. 



In the years following the publication of the cell theory there 

 was an almost total lack of understanding as to how cells arise. 



