258 THE BODY AT WORK 



which has, it is needless to say, no effect upon the shape of the 

 cell (cf. p. 9). 



The unicellular animal amoeba, leucocytes, and certain 

 spores of plants, are devoid of cell-wall (cf. p. 28). Their soft 

 protoplasm is not limited by a rigid case. When it streams, 

 the form of the cell is changed. True, we must not think of the 

 body-substance of an amoeba as homogeneous. It exhibits an 

 internal structure. Yet its architecture is not, so far as we 

 can see, sufficiently fixed to restrict the directions in which 

 it can stream. Any change of shape is possible. We can- 

 not find in Nature an isolated clump of living protoplasm ; 

 nor do we suppose that, if we found it, it would prove to 

 be homogeneous. It appears to be necessary that proto- 

 plasm and metaplasm the terms have no chemical signifi- 

 cance ; " primary " and " secondary," or " chief " and " sub- 

 sidiary " would be equally distinctive should be intermixed. 

 Streaming is apparently due to alterations in the surface 

 relations of the two substances. 



In multicellular animals certain elongated cells are arranged 

 in groups, with their long axes all pointing in the same direc- 

 tion. They can change in shape, diminishing in length, with 

 equivalent increase in breadth. Since all the cells of a group 

 undergo this change of form at the same time, the result is an 

 alteration in the shape of the animal of which they are a part. 

 Applying the experience which we have gained in studying the 

 movements of unicellular organisms, we conclude that these 

 elongated cells are composed of two substances proto- 

 plasm and metaplasm. The restriction of their capacity 

 for altering their shape to one direction indicates that their 

 protoplasms and metaplasm are not indifferently mixed. 

 The two substances set in lines in the direction of the 

 long axis of the cell. Hence, when streaming occurs when 

 the force which keeps the molecules of protoplasm and 

 of metaplasm in their respective rows is relaxed the lines 

 thicken. The cell broadens, with an equivalent diminution of 

 length. 



Muscle-fibres exhibit all degrees of specialization. The 

 simplest, " plain muscle-fibres," are found in the wall of the 

 alimentary canal, of bloodvessels, of ducts, in the tissue of the 

 spleen, in the skin, and elsewhere. Each fibre is a fusiform 



