CYTOLOGY 15 



The portion of the cytoplasm which is enclosed by these two layers 

 shows the granular appearance previously mentioned, and is known as the 

 endoplasm. This part of the cytoplasm is differentiated into three com- 

 ponent systems. The outer portion (the ectoplasm of Amoeba) next to the 

 hyaline layer, is elastic and almost solid (geloid) in texture, while the inner 

 portion is definitely fluid and much less cohesive than the outer layer. 



When protoplasm is immersed in water the external layer remains 

 coherent, but if this is ruptured the internal portion mixes readily with 

 the water. The external membrane appears to consist of a mixture of lipin 

 and protein molecules, and is probably of molecular thickness, but it is the 

 controlling agent in determining the entry of substances into the cell or, 

 as it is called, the protoplasmic permeability. For example, the dye, 

 Eosine, will not enter a living cell, but if it is injected through the proto- 

 plasmic membrane it diffuses readily in the liquid endoplasm. 



A third component, called kinoplasm, has been distinguished, con- 

 sisting of a system of very delicate fibrils, only visible with difficulty, but 

 distinguishable by the streams of granules which they carry along. The 

 kinoplasm appears to be connected to both the inner and outer protoplasmic 

 membranes. 



It would be a mistake to view these protoplasmic systems as fixed and 

 permanent structures. The most essential characteristic of protoplasm is 

 its changeability, and not only do reversible alterations occur in the different 

 components but probably a continuous exchange of material occurs between 

 them. 



The living quality of protoplasm lies in the interaction of its components, 

 not in the properties of any individual part of it. It is an active system, 

 neither a substance nor simply a mixture of substances, but a chemical 

 machine in which all the components have a part to play which may be 

 more or less essential to the whole. 



The vacuoles are part of the cytoplasmic system, though in a mature 

 plant cell only one large vacuole may be present, occupying the whole central 

 part of the cell and confining the cytoplasm to a thin peripheral layer, the 

 protoplasmic utricle. In the youngest cells, however, either no vacuoles 

 occur or they are very small, and one of the main features of the development 

 of cells towards maturity is the appearance of vacuoles in the cytoplasm, a 

 process called vacuolization. It is still an open question if vacuoles arise 

 de novo in cells, but the weight of evidence seems to be on the side of the 

 view which regards them as always present, even in meristematic cells. Large 

 vacuoles are probably derived chiefly from the growth and fusion of small 

 pre-existing ones. Passing through a phase which suggests a network of 

 canals, possibly due to cytoplasmic streaming movements, these separate 

 vacuoles eventually fuse into larger units or even finally into one. The 

 cell sap, which the vacuoles contain, is an assemblage of practically all the 

 materials which figure in metabolism, from inorganic salts to enzymes and 

 even proteins. Some of these, notably the sugars and organic acids, are 

 highly osmotic, and are the cause of the osmosis by which water is drawn 



