24 CELLS. 



and attraction-sphere have been found in so many cells that they 

 may be regarded as essential constituents of every cell. 



Protoplasm. This is the principal part of a cell, and is of an 

 albuminous nature. Chemically it consists of water (75 per cent, 

 or more), proteids, lecithin, cholesterin, and phosphates and chlo- 

 rids of sodium, potassium, and calcium, and sometimes fat and 

 glycogen. Microscopically examined it is found to be made up of 

 spongioplasm and hyaloplasm. 



Spongioplasm. tinder high powers of the microscope the pro- 

 toplasm of a cell presents the appearance of a fine network, called 

 reticulum, spongework, or spongioplasm. This network has in it 

 knots, which give to it a granular appearance. These knots or 

 granules are of the same chemic nature as the network that is, 

 are albuminous or proteid. It is still undecided whether these 

 granules are constituent parts of the protoplasm or are its products. 

 Collectively they are denominated granuloplasm. Other granules 

 may be present which are not connected with the network, and 

 which are not proteid in character, but fatty or starchy or con- 

 tain coloring-matter. In some instances they are of an inor- 

 ganic nature. Granules of this latter kind constitute paraplasm ; 

 by which is meant any and all material contained in a cell, not 

 being an actual part of it, whether there as pabulum or food for 

 the cell, or as waste material to be excreted. 



Hyaloplasm. In the meshes of the spongioplasm is the hyalo- 

 plasm, a clear substance differing but slightly in its consistence 

 from the spongioplasm, although it is less solid. 



Ameboid Movement. Protoplasm is endowed with the power 

 of motion, which from its resemblance to the motion of the ameba, 

 a minute animal, which is but a mass of protoplasm, is called 

 ameboid. Examined under the microscope the ameba puts out 

 from its sides projections of its protoplasm pseudopodia ; and 

 later the whole mass flows into one or more of these projections, 

 thus changing its position and its shape. This ameboid movement 

 takes place in the white blood-corpuscle, and in some other cells 

 as well as in the ameba. The pseudopodia are frequently drawn 

 back into the protoplasm, or retract, thus illustrating the posses- 

 sion by the protoplasm of contractility. Their formation is due to 

 an outflowing of the hyaloplasm, and their retraction to return 

 of the hyaloplasm to the interstices of the reticulum. Ameboid 

 movement is said to be spontaneous ; but if so, it can also be pro- 

 duced by the action of heat, by dilute solutions of salt, by mod- 

 erate currents of electricity, and by many other agents, all of 

 which are called stimuli, because of their power to stimulate this 

 movement. On the other hand, certain agents have the power of 

 stopping or inhibiting the movement if it has begun. Thus a 

 temperature above 40 C. or below C. acts as an inhibitant, 

 while if the high temperature is continued the protoplasm is coag- 



