156 



THE CELL 



formation in the cell-bod)-, but which can only be produced by 

 division. Ev r en in the youngest plant-cells, according- to their 

 opinion, minute vacuoles are present, which multiply continually 

 by fission, and which are distributed amongst the daughter cells 

 when cell division occurs. Here all the vacuoles of the whole 

 plant would originate from those of the meristem. This theory 

 however is disputed by other investigators. Just as the proto- 

 plasm is bounded externally by a peripheral layer, the vacuoles, 

 in de Vries' opinion, possess a special wall (the tonoplast), which 

 regulates the secretion and accumulation of the dissolved sub- 

 stances present in the cell sap. 



Na cv 





fck\ h- i WSJ 



ML? TX*&&* 



: i^MJJL 



Fia. 63. Actinosplwerium. EicTihorni (after R. Hertwig, Zoologie, Fig. 117): M medullary 

 substance with nuclei (n) ; R peripheral sibstmce with contractile vacuoles {cv); Na 

 nutrient material. 



The formation of vacuoles also occurs to a considerable extent 

 in the lower organisms. In Actinosphcerium, for example, the 

 protoplasmic body has quite a foamy appearance, in consequence 

 of the large number of great and small vacuoles present in it. 

 A few vacuoles, the number of which is constant, acquire a 

 specially contractile peripheral layer; they are then described as 



