CHAPTER III 

 THE STRUCTURAL COMPONENTS OF PROTOPLASTS 



In the previous chapter the fact that the cell is not always a distinct 

 element in tissues was emphasized as one that should aid in clarifying our 

 conceptions of factors responsible for the action of the complex organism 

 as a whole, especially during its ontogeny. Nevertheless in most organ- 

 isms and tissues the cell does appear clearly as a more or less standardized 

 unit having cytoplasm, nucleus, and membranes. In addition to these 

 there may be other diverse functional and structural differentiations that 

 cause some cells to look very unlike "typical " protoplasts. We shall now 

 extend our description of the protoplast by reviewing in more detail its 

 principal internal features. 



Cytoplasm. — The cytoplasm consists ordinarily of an optically clear, 

 somewhat viscous fluid {hyaloplasm) in which there may be embedded a 

 multitude of small droplets and granules that give it a visible structure. 

 The fundamental structure necessary to the performance of vital activi- 

 ties is known to lie in the hyaloplasm beyond the limit of visibility with 

 the photomicroscope; this matter will be considered further in the next 

 chapter. The cytoplasmic portion of the protoplast is called the cytosome. 



The cytosome is often differentiated into fairly distinct regions differ- 

 ing in viscosity. In many cells the more viscous portion, the plasmagel, 

 forms a cortical layer of variable thickness just beneath the plasma mem- 

 ])rane, while the more fluid portion, the plasmasol, lies farther in. The 

 cytoplasm may change rapidly and locally from one state to the other, 

 and it is evident that such alterations play a role in ameboid locomotion 

 and the streaming observed in tissue cells. 



In embryonic or meristematic tissues the cytoplasm seems to be less 

 differentiated than the other cell components enclosed within it and more 

 like what we may imagine primitive protoplasm to have been. It is, 

 however, capable of differentiation in many ways. In muscle tissue, for 

 example, it may be almost completely transformed into fine longitudinal 

 myofibrils that function somehow in the act of contraction (Fig. 11). 

 The cytoplasm of nerve cells has delicate neurofibrils probably concerned 

 in the conduction of stimuli (Figs. 12, 13). In the protozoa the cytoplasm, 

 or at least its external hyaline portion (ectoplasm), may develop loco- 

 motor extensions in the form of undulating membranes, cilia, and flagella 

 of many types. The pseudopodia formed by amebas and mj^xomycetes 



22 



