PROTOPLASM 5 



B. The Structure of Cells 

 1. Cytoplasm 



The many different types of cells, although showing considerable 

 variation in certain details, nevertheless exhibit a number of 

 fundamental structural features which may now be noted. In 

 the first place the protoplasm is differentiated into the cytoplasm, 

 which constitutes the main mass of the cell body, and a much 

 smaller spherical body, the nucleus, which typically lies near the 

 center of the cytoplasm. This differentiation of the cell protoplasm 

 is regarded as basic for cell structure, as is shown by the usual 

 definition that a "cell is a mass of protoplasm containing a nu- 

 cleus." Cytoplasm and nucleus together are often termed the 

 protoplast. In many of the unicellular animals the cytoplasm 

 shows a definite division into a larger central mass, the endoplasm, 

 in which the nucleus is embedded, and an outer portion, the 

 ectoplasm. (W. f. 7.) 



The cytoplasm contains certain protoplasmic inclusions, such as 

 mitochondria, golgi rodies, plastids, etc., which undoubtedly 

 play very important roles in the life processes of cells. Almost all 

 of these bodies are still the subjects of extended research bearing 

 upon their structure and functions. The plastids are of particular 

 interest because a certain type, the chloroplastids, found in the 

 green plant cells contain the basic food-building substance, chlo- 

 rophyll, to be considered in detail later. Another important 

 protoplasmic inclusion usually present in the cytoplasm is the 

 centrosome which is very active in cell division and, in fact, is 

 generally regarded as the dynamic center for this fundamental 

 process. 



However, not all of the inclusions in the cytoplasm consist of 

 living matter, for various non-living materials, collectively termed 

 metaplasm, are present. The metaplasm varies greatly in char- 

 acter and amount in different types of cells and in the same cell at 

 different times. It consists of waste products and reserve food 

 materials, and it may be in the form of water vacuoles, fat droplets, 

 crystals, etc. Mature plant cells generally possess a large central 

 cavity, the cell vacuole, which is filled with a fluid, the cell 

 sap, containing materials of importance in the nutrition of the cell. 

 (W. f. 10.) 



