34 THE CELL 



membrane, has been proved beyond a doubt by the following 

 observation, which has played such an important part in the 

 history of the cell theory (I. 7). In many algse (fficlogonium, 

 Fig. 14), at the time of reproduction, the protoplasmic substance 

 becomes detached from the cellulose cell- wall, and, whilst parting 

 with some of its fluid contents, contracts up into a smaller volume, 

 so that it no longer quite fills up the cavity ; it thus forms a 

 naked swarmspore, which is either globular or oval in shape (A). 

 After a time this swarmspore breaks down the original cell-wall, 

 and, escaping through the opening it has made, reaches the 

 exterior. It then develops cilia (0) upon its surface, by means of 

 which it moves about pretty quickly in the water, until after a 

 time it comes to rest (D), when it differentiates a delicate new 

 membrane upon its surface. Thus Nature herself has afforded us 

 the best evidence that the protoplasmic body is the true living 

 elementary organism. 



A similarly great formation of vacuoles and separation of sap, as 

 is found in plant-cells, is also seen in the naked protoplasm of the 

 lower unicellular organisms, especially in certain Reticularia and 

 Radiolarians ; thus the Actinosphcerium, which is depicted in Fig. 

 15, presents quite a frothy appearance, resembling the fine froth 

 which is produced when albumen or soap-suds are beaten up. An 

 immense number of larger and smaller vacuoles, filled with fluid, 

 are distributed throughout the whole body. These are only 

 separated from one another by delicate partition walls of proto- 

 plasm, which are sometimes too thin to be measured. The 

 protoplasm consists of a homogeneous ground substance, in which 

 granules are embedded. 



The result of this formation of vacuoles is that the protoplasmic 

 substance becomes broken up, so that surfaces of it become exposed 

 to the nutrient solutions in the vacuoles, in consequence of which 

 diffusion can take place between them. Evidently the whole 

 arrangement adds considerably to the facility with which 

 materials are taken up and given out. This internal increase of 

 surface may be compared with the external increase of surface, 

 which is shown in the formation of many-branched pseudopodia 

 (Fig. 10), and indeed it answers the same purpose. 



In animal-cells, on the contrary, the formation of vacuoles and 

 the secretion of sap only take place extremely rarely, for instance, 

 in notochordal cells; on the other hand, adventitious substances, 

 such as glycogen, mucin, fat globules, albuminous substances, etc., 



