1074 GENERAL ANATOMY OR HISTOLOGY. 



ular appearance, under a high power of the microscope, is seen to be due to the fact 

 that protoplasm consists of a network or honeycombed reticulum, containing in its 

 meshes a homogeneous substance. The former is known as spongioplasm, the 

 latter as hyaloplasm. The granular appearance is often caused by the knots of 

 the network being mistaken for granules ; but, in addition to this, protoplasm 

 often contains true granules, some of which are proteid in nature and probably 

 essential constituents; others are 'fat- or pigment-granules, and are regarded 

 as adventitious material taken in from without. The size and shape of the 

 meshes of the spongioplasm vary in different cells and in different parts of the 

 same cell. In many fixed cells, e.g., epithelial cells, the external layer becomes 

 denser than the rest, and often altered by the deposition in it of some chemical 

 substance, so as to constitute a membrane which encloses the rest of the proto- 

 plasm and forms the cell-wall. The relative amount of spongioplasm and hyalo- 

 plasm varies in different cells; the latter preponderating in the young cell 

 and the former increasing in anlount, at the expense of the hyaloplasm, as the 

 cell grows. 



The most striking characteristics of protoplasm are its vital properties of motion 

 and nutrition. By motion is meant the property which protoplasm has of changing 

 its shape and position by some intrinsic power, which enables it to thrust out from 

 its main body an irregular process, into which the whole of the protoplasmic sub- 

 stance is gradually drawn, so that the mass comes to occupy a new position. This, 

 on account of its resemblance to the movements observed in the Amoeba or Proteus 

 animalcule, has been termed "amoeboid movement." Ciliary movement, or the 

 vibration of hair-like processes from the surface of any structure, may also be 

 regarded as a variety of the motion with which protoplasm is endowed. 



Nutrition is the power which protoplasm has of attracting to itself the materials 

 necessary for its growth and maintenance from surrounding matter. When any 

 foreign particle comes in contact with the protoplasmic substance, it becomes incor- 

 porated in it by being enwrapped by one or more processes projected from the 

 parent mass which enclose it. When thus taken up, it may remain in the sub- 

 stance of the protoplasm for some time without change, or may be again extruded. 



The nucleus is a minute body, imbedded in the protoplasm, and usually of a 

 spherical or oval form, its size having little relation to the size of the cell. It 

 is surrounded by a well-defined wall, the nuclear membrane, which encloses the 

 nuclear contents. These are known as the nuclear substance (nuclear matrix), 

 which is composed of a homogeneous material and a stroma or network. The 

 former is probably of the same nature as the hyaloplasm of the cell, but the latter, 

 which forms also the wall of the nucleus, differs from the spongioplasm of the cell- 

 substance. It is sometimes known as the chromoplasm or intranuclear network, 

 and consists of a network of fibres or filaments arranged in a reticular manner. 

 These filaments stain very readily with certain dyes ; they are therefore named 

 chromatin ; while the interstitial substance does not stain readily, and is hence 

 called achromatin. In some resting nuclei, i. e., nuclei which are not undergoing 

 subdivision, the nuclear filaments do not form a network, but present the appear- 

 ance of a convoluted skein, similar to that found in a nucleus about to undergo 

 division, and which will be immediately described. 



Within the nuclear matrix are one or more highly refracting bodies, termed 

 nucleoli, connected with the nuclear membrane by the nuclear filaments. They 

 are regarded as being of two kinds. Some are mere local condensations of the 

 chromoplasm; these are irregular in shape and are termed pseudo-nucleoli ; 

 others are distinct bodies differing from the pseudo-nucleoli both in nature and 

 chemical composition ; they may be termed true nucleoli, and are usually found 

 in resting cells. 



The nuclear substance differs chemically from ordinary protoplasm in containing 

 nuclein, in its power of resisting the action of acids and alkalies, in its imbibing 

 more intensely the stain of carmine, hsematoxylin, etc., and in its remaining 

 unstained by some reagents which color ordinary protoplasm. 



