THE STRUCTURAL BASIS OF THE BODY 17 



spherical body lying near the centre of the cell and bounded by a definite 

 contour or nuclear membrane. In its interior it contains masses or filaments 

 of a material known as chromatin, which are strung, so to speak, on a fine 

 network of material known as linin. Besides the granules of chromatin, 

 other masses are sometimes found which stain in a different manner and are 

 called nucleoli. The material filling up the meshes of the network is the 

 nuclear sap or nucleoplasm. The cytoplasm, which varies greatly in extent 

 in different cells, varies also in its appearance, being sometimes homogeneous, 

 sometimes alveolar, sometimes granular in structure. In it can be often 

 distinguished differentiated parts which may be regarded as organs of the 

 cell. Thus in the amoeba we have the contractile vacuoles already men- 

 tioned. In the green parts of plants the cytoplasm contains green granules, 

 the chloroplasts, whose special function it is to assimilate carbon dioxide from 

 the atmosphere, and by means of the energy of the sun's rays to convert this 

 into starch with the evolution of oxygen. Other parts of the plant have 

 similar granules, the leucoplasts, whose office it is to build up sugar into 

 starch, and it is possible that other kinds of these c plastids ' with special 

 chemical functions are present in the cytoplasm of many cells. In addition 

 to these cell organs, the cytoplasm may contain granules which represent 

 stages in the metabolism of the cell and are either food material which is 

 being assimilated or products of the disintegration of the protoplasm, 

 formed either for the service of the cell itself or, in the case of the multi- 

 cellular animals, for the service of other cells of the organism. Others 

 of these granules may represent reserve material, i.e. excess of nourishment 

 which has been put aside by the cell in an insoluble form, to serve for its 

 subsequent needs in times of scarcity. 



THE PHYSICAL STRUCTURE OF PROTOPLASM. Owing to the close 

 similarities which exist between the fundamental properties of all living 

 organisms, histologists have sought to discover some corresponding uniform 

 morphological organisation of the physical basis of these phenomena, namely, 

 protoplasm. 



It is often impossible, even under the highest powers of the microscope, 

 to make out any structure whatsoever in the cytoplasm, which is spoken of 

 then as hyaline. In most cases examination of a cell, even unstained, shows 

 some differentiation between a more or less regular framework or meshwork 

 and a more fluid portion filling up its interstices, and these appearances are 

 still more manifest when the cells have been fixed by various hardening 

 fluids. All the results obtained in this manner must be regarded with some 

 suspicion, since, as has been shown by Fischer and by Hardy, it is possible to 

 imitate artificially the various structures, which have been assigned as 

 characteristic of protoplasm, by hardening a homogeneous colloidal solution 

 such as egg-white by different methods and with different agents. The 

 theories of protoplasmic structure can be classified under three heads : 



1. THE GRANULAR. THEORY OF ALTMANN. By the use of certain hardening 

 reagents, a dense mass of spherical or rod-shaped granules may be demon- 

 strated in almost all cells of the body (Fig. 4). These granules have been 



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