THE ANIMAL CELL. 



[GH. II. 



in the hairs of the stinging-nettle and Tradescantia and the cells 

 of Vallisneria and Chara ; it is marked by the movement of the 

 granules nearly always imbedded in it. For example, if part of a 

 hair of Tradescantia (fig. 15) be viewed under a high magnifying 

 power, streams of protoplasm containing crowds of granules 

 hurrying along, like the foot-passengers in a busy street, are seen 

 flowing steadily in definite directions, some coursing round the 

 film which lines the interior of the cell-wall, and others flowing 

 towards or away from the irregular mass in the centre of the 

 cell-cavity. Many of these streams of protoplasm run together 

 into larger ones and are lost in the central mass, and thus ceaseless 

 variations of form are produced. The movement of the proto- 

 plasmic granules to or from 

 the periphery is sometimes 

 called vegetable circulation, 

 whereas the movement of 

 the protoplasm round the 

 interior of the cell is called 

 rotation. 



The first account of the 

 movement of protoplasm 

 was given by Rosel in 1755, 

 as occurring in a small 

 Proteus, probably a large 

 freshwater amoeba. His 

 description was followed 

 twenty years later by Corti's 

 demonstration of the rota- 

 tion of the cell sap in Characese, and in the earlier part of this century 

 by Meyer in Vallisneria, 1827, and by Robert Brown, 1831, in 

 "Staminal Hairs of Tradescantia." Then came Dujardin's descrip- 

 tion of the granular streaming in the pseudopodia of Rhizopods ; 

 movements in other animal cells were described somewhat later 

 (Planarian eggs, v. Siebold, 1841 ; colourless blood-corpuscles, 

 Wharton Jones, 1846). 



There is no doubt that the protoplasmic movement is essentially 

 the same thing in both animal and vegetable cells. But in 

 vegetable cells, the cell- wall obliges the movement to occur in the 

 interior, while in the naked animal cells the movement results in 

 an external change of form. 



Although the movements of amoeboid cells may be loosely 

 described as spontaneous, yet they are produced and increased 

 under the action of external agencies which excite them, and 

 which are therefore called stitmdi, and if the movement has ceased 



Fig. 15. Cell of Tradescantia drawn at successive 

 intervals of two minutes. The cell-contents 

 consist of a central mass connected by many 

 irregular processes to a peripheral film, the 

 whole forming a vacuolated mass of proto- 

 plasm, which is continually changing its 

 shape. (Schofield.) 



