MANIFESTATIONS OF VITALITY 17 



protoplasm appears like an aggregate of bubbles in a frothy soap 

 suds, with the walls of the bubbles or, in protoplasm, alveoli, 

 relatively dense and the intra-alveolar substance relatively 

 more fluid; in still other cases the more refringent parts are in 

 the form of minute rods or nbrillae surrounded by a more fluid 

 matrix. In all forms assumed by protoplasm, there are in- 

 variably fine granules, called microsomes, scattered throughout. 



These different types of protoplasmic structure have given 

 rise to different theories as to the physical make-up of proto- 

 plasm, and the adherents of each theory hold that all other 

 appearances are only modifications of the structures which they 

 believe fundamental. Thus we find biologists who hold to the 

 "reticular' : theory, others who hold the "alveolar" theory, 

 others again who adhere to the "fibrillar' : theory, and still 

 others who maintain that all apparent structures are secondary 

 and unimportant and that the only vital elements in the physi- 

 cal make-up are the granules or microsomes. Whatever may 

 be the outcome of disputes over the relationship of the differ- 

 ent appearances the fact remains that protoplasm consists of 

 an aggregate of fluid-like substances of different densities which 

 may assume a variety of configurations. 



FORM.- -These appearances, even if uniform, could not be 

 relied upon as a sure manifestation of vitality. Lifeless protein, 

 albumen, and even emulsions of oil, water and salt, give similar 

 appearances so that other manifestations must be taken con- 

 jointly. Appearance combined with form gives fairly definite 

 evidence of life. Form, however, is closely connected with the 

 configuration of morphological units of protoplasmic structures. 

 With the exception of a small number of amorphous living 

 things, all types of animals and plants have a definite and recog- 

 nizable form. No living thing consists of a homogeneous sheet 

 or column or ball of semi-fluid protoplasm, but in all higher 

 types the protoplasm is divided among myriads of very tiny 

 units called CELLS which may become differentiated in the great- 

 est variety of ways. The few amorphous types of living things 

 consist, as a rule, of but one single cell (e.g., Amoeba, Fig. 10). 

 In life the individual cells of an animal or plant cannot be readily 



