GENERAL ANATOMY 



53 



urchin; that a species of amoeba always performs only the movements 

 characteristic of tha,t species, we must assume that the functioning part 

 of this cell, the protoplasm, has in each case its peculiarities. We are 

 driven to the assumption of an almost unlimited diversity of protoplasm, 

 even if we concede an important share in the prominent differences to 

 the nucleus, of which we shall speak later. 



General Properties of Protoplasm. — The similarity of protoplasm 

 expresses itself in its appearance and in its vital phenomena. Under slight 

 magnification, protoplasm appears as a faintly gray substance (sometimes 

 colored yellowish, reddish, etc., by pigments) in which numerous strongly- 

 refracting granules are imbedded. The vital characteristics of this sub- 

 stance are movement, irritahility, power of assimilation and of reproduction. 



By using higher powers a finer structure can be seen in the 'homogeneous 

 protoplasm' of earher writers. It looks like a iine-meshed framework {filar 

 substance, spongioplasm, cell retictilwn) the interstices of which are filled with 

 other material (interfilar substance, enchylcma, ground substance). The question 

 whether this framework is formed of threads and trabeculae or whether the 

 appearance is not formed by small cham- 

 bers, bounded by fine partition-walls 

 (foam structure of protoplasm), such as 

 results when two fluids which do not mix 

 (like olive oil and soda solution) are 

 shaken together until a very fine froth 

 is produced. This view that protoplasm 

 has a foam structure e.xplains how it can 

 be a fluid aggregate with a fine structure. 

 Regarding the fluid aggregate condition cf 

 protoplasm (long called a 'solid-fluid') 

 there has been much dispute. Exact re- 

 searches regarding its physical condition 

 show that it behaves hke a fluid. 



Movement of Protoplasm. — Move- 

 ment expresses itself first in changes of 

 form of the whole body — amoeboid move- 

 ment — and secondly in the change of 

 position of the small granules in the 

 interior of the protoplasm — streaming 

 of granules. Examples of amoeboid 

 movement (fig. i6) are found in many 

 Protozoa, and' the colorless blood-cells (leucocytes) of multicellular ani- 

 mals; here the protoplasmic body sends out coarser and finer processes, 

 which may be again withdrawn, serving for locomotion and hence called 

 pseudopodia. The streaming of granules can be observed in the interior 

 of the cell-body, as well as in the pseudopodia extending from this. The 

 pseudopodia may even be so fine as to be at the Umits of visibihty with our 



Fig. i6. — Amcrba proteus (after 

 Leidy). ek, ectosarc; en, entosarc; 

 cv, contractile vacuole; n, nucleus; 

 A'', food-vacuoles. 



