MANIFESTATIONS OF VITALITY 17 



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 fibrillae 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 proteid, 

 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 (examples in Figs. 2, 3, 4). 

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

 made out; some of them are glandular in function, others are 



