18 ESSENTIALS OF ZOOLOGY 



Material is taken up and is being discharged, and these pro- 

 cesses are accompanied by the gain and loss of energy. These 

 considerations lead to a deeper enquiry into the constitution 

 of the cell, its chemical composition and physical state. 

 Chemically, protoplasm is regarded as a mixture of highly 

 complex carbon compounds carbon combined with nitrogen, 

 hydrogen, and oxygen, with iron, sulphur, and phosphorus 

 in a colloidal state. From the examples we have studied it is 

 evident that physically the colloids are resolved into a more 

 or less firm gel and a sol containing granules. The ectoplasm 

 of Amoeba is an incipient gel, a gel which becomes more dis- 

 tinct in Flagellata and especially in Infusoria. The endo- 

 plasm is a sol, it behaves as a sol, and is in a constant state of 

 metabolic fluctuation. Turgidity is produced by water which 

 distends the ectoplasmic skeleton of the cell, and the water 

 associated with protoplasm may be regarded as water of 

 colloidisation. Water is present also in vacuoles, and in some 

 cells to such an extent as to reduce the protoplasm to threads 

 of extreme tenuity. It is probable that the cell sap of the 

 vacuoles only is lost in desiccation. The vacuoles are in a 

 state of surface tension, and between them the protoplasm is 

 constantly streaming in the spaces at its disposal. It has 

 been noted already that in addition to water carrying material 

 in solution, other inclusions may be found in protoplasm. 



The nucleus is similar, but it may be looked upon as an 

 assemblage of colloids of somewhat different composition, 

 which forms a membrane in contact with the cytoplasm, the 

 nuclear membrane. It is a fluid which is the medium of im- 

 portant chemical changes. It may give rise to a framework 

 of linin, and it contains or may produce a substance character- 

 ised by its staining properties and called chromatin. The 

 chromatin is carefully divided during cell division, and this 

 manifestation of mitosis has naturally led to the conclusion 

 that the chromatin transmits thereby inherited qualities. At 

 present, however, it is just as well to observe that the whole 

 cell is carefully orientated during division, and that the whole 

 plasm, cyto- as well as nuclear, is divided. 



Ordinary chemical analysis does not indicate the differences 

 in the composition of the protoplasm from different sources, 



