10 INTKODUCTION PEOTOPLASM CELL 



every type of cells, at certain, perhaps all, stages of development and 

 active function. They have been seen and studied even in living plant 

 cells (Maximow), and in animal cells grown in 

 artificial media they were observed by M. E. and 

 W. H. Lewis (Amer. Jour. Anat., 17, 3, 1915) 

 to move, to change shape, to divide into granules 

 and again to reunite into filaments facts which 

 render inadmissible their interpretation in fixed 

 material as lipoid precipitation products (Faurier- 

 Fremier), and strongly suggest their connection with 

 FIG 11 INTRA- me tabolic activity. They have the chemical composi- 

 CELLULAR NET- tion of a lipoid (probably a phosphatid) united to an 

 WORK, OR 'TROPH- albuminoid base. It has been suggested that they are 



OSPONGIUM,' WITH- _ & 



IN A PURKINJE a su PP r t to, and the region of, oxidation. Certain 

 CELL OF THE CERE- investigators (Benda, Meves, Duesberg) regard them 

 BELLUM OF STRix as t ne cytoplasmic basis of heredity, and ascribe to 



FLAMMEA. . . . . 



Golgi's stain. (After them an im P ortant role in histogenesis. 



Golgi.) Schreiner's findings in developing fat cells sug- 



gest a nutritive significance (vide, p. 64). 



STRUCTURE OF PROTOPLASM 



Four main types of protoplasmic structure are generally recognized: 

 (1) the homogeneous; (2) the granular; (3) the alveolar or foam type; 

 and (4) the fibrillar, (a) reticular or sponge type and (b) filar (Fig. 

 12). The type of protoplasm of a particular cell may vary with the 

 stage of development and function. In successive stages of development 

 and differentiation the protoplasm of the same cell may pass from the 

 apparently homogeneous, through the granular and granulo-alveolar, to 

 a granulofibrillar type. Homogeneous protoplasm, as for example the 

 ectoplasmic layer of ameba, is more probably to be interpreted as com- 

 posed of minute, perhaps ultramicroscopic, colloidal granules ('colloidal 

 biogens'). Young and undifferentiated cells commonly have a granular 

 cytoplasm. In general it may be said that the actual fundamental type 

 of protoplasm is the granular. This changes into the alveolar type by 

 the appearance of spherules among the granules. The contents of the 

 alveoli constitute the alveolar substance; the inter-alveolar sap, the hya- 

 loplasm ; the granules, the microsomes ; and the walls of the alveoli may 

 be identified with the spongioplasm of the reticular type of protoplasm. 

 Another interpretation of alveolar protoplasm regards the content of the 



