CHAPTER II 

 THE CELL 



PROTOPLASM, as it is ordinarily encountered in living things, is always 

 organized as certain structural units in which it shows some characteristic 

 differentiation. The cell is such a working unit of protoplasm (see Fig. 

 i). These living protoplasmic units are the structural units of which all 

 organisms are formed. Living units of an extremely low type have been 

 described, which consist of simple undifferentiated protoplasm. These 

 forms are not very frequently encountered, and it is probable that the 



lack of differentiation 

 is due to the ineffi- 

 ciency of the methods 

 employed in their 

 study. 



The simplest of 



FIG. 4. Tetramitus chilomonas, a unicellular animal which these Cells are reDre- 

 shows the nuclear material in a distributed condition; chr., 

 chromatin masses. From CALKINS. Sented in SOHie low 



forms of plants and 



animals (Fig. 4). In these low forms a certain differentiation of the 

 protoplasm is detected by staining processes. Scattered throughout 

 the general protoplasm of such a unit are numerous rounded granules 

 which stain deeply, while the remaining substance does not stain. The 

 granules, because of their affinity for stain, have been called chromatin. 

 The non-staining part of such a structural unit is spoken of as cytoplasm. 

 A structural unit or cell of this character, which has chromatin diffused 

 or scattered throughout its cytoplasm, is called by some a pseudocyst 

 or false cell. Figure 4 shows such an animal in which the chromatin 

 appears distributed throughout the cell. 



Except for these low forms of life, the unit of structure is always a 

 more highly specialized mass of protoplasm. Part of the protoplasm of 

 these higher cells has been differentiated to form a specific mass that 

 always is found within the less highly specialized protoplasm. This 

 is known as the nucleus, and the protoplasm within which it lies receives 

 the term cytoplasm. The nucleus itself is clearly defined as a round 

 to irregularly shaped body, more dense than the cytoplasm. It is by no 

 means homogeneous. The modified protoplasm or karyoplasm of which 



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