22 FUNDAMENTAL PRINCIPLES 



the orderly sequence of reactions which takes place in living matter and 

 makes possible growth by intussusception. 



6. The salts in protoplasm are to a very great degree ionized, and the 

 state of ionization contributes to the speed of chemical reactions within the 

 mass. 



29. Microscopical Structure of Protoplasm. — The droplets of the 

 disperse phase in protoplasm are mostly too small to be within the range 

 of microscopic vision, and those which are not too small are hardly visible. 

 The structure of protoplasm as exhibited under the microscope, however, 

 reveals the presence of firm granules of different sizes ; of fibers, which may 

 form a network, or reticulum; and of droplets, or alveoli. None of these 

 is evidence of the emulsoid nature of protoplasm but all may have a bear- 

 ing on the function of the cell. Emphasis upon one or another of these 

 different elements has formed the basis for three theories of the normal 



Fig. 3. — Semidiagrammatic sketches illustrating the different appearances exhibited 

 by protoplasm. A, the granular type; represents cells from the liver of the mouse. B, the 

 fibro-reticular type; represents a nerve cell with its fibers cut away. C, the alveolar type; 

 represents a portion of alveolar protoplasm. (Figs. A and C modified from Wilson, The Cell, 

 A, after Altmann, by the courtesy of The Macmillan Com,pany.) 



structure of protoplasm, which has been thought to be (1) granular, (2) 

 fibrillar or reticular, or (3) alveolar. Though most protoplasm now seems 

 to be alveolar in character, a granular appearance is often exhibited in 

 gland cells, and fibers are prominent in nerve cells, muscle cells, and some 

 epithehal cells (Fig. 3). 



30. Appropriateness of Protoplasm as Living Substance. — It is 

 evident from what has been said as to the chemical and physical char- 

 acteristics of protoplasm that among these are many which contribute 

 directly to the carrying out of life activities, and all are necessary in a 

 substance in which life can manifest itself. Its physical characteristics, 

 its chemical nature, its organization, its proneness to change, its ability 

 to assume an almost infinite variety of forms, and its capacity constantly 

 to carry on metabolism all make protoplasm the only appropriate living 

 substance. Its exceedingly great complexity offers the possibility of 

 almost infinite internal change and adjustment, while at the same time 



