THE CELL. 19 



Here the cells are joined together only for certain functions : 

 for example, to cover and serve as a protection, to separate, to 

 absorb, to draw together, or to conduct impulses. In unicellular 

 organisms, on the contrary, a cell is a complex of organs which 

 serve different functions. 



The essential constituents of the cell are 



(a) the protoplasm and 



(b) the cell nucleus. 



The nucleus may in many cases disappear, especially if the 

 cell begins to lose its vital activity. 



Protoplasm is a morphological conception, and not a body 

 capable of sharp definition chemically. By the term " proto- 

 plasm " is not to be understood a uniform substance with con- 

 stant physical and chemical properties, but, on the contrary, a 

 combination of various different chemical bodies joined with 

 one another in a truly wonderful way ; a substance which 

 exhibits different physical, chemical, and biological properties 

 (O. Hertwig). Protoplasm is semifluid, elastic, almost always 

 colorless, and insoluble in water. It is not entirely homogene- 

 ous, but shows fine granules (microsomes) and fibrils which are 

 contained in the homogeneous ground substance. 



We may often observe that the cell consists at the periphery 

 of a non -granular protoplasm (hyaloplasm), while in the inner 

 part there is a granular protoplasm-mass (granuloplasm}. These 

 two parts of the cell are known also as ectoplasm and endoplasm. 

 respectively. 



The chemical composition of protoplasm is unknown, except 

 that its essential and most important constituent belongs to the 

 protein substances (albuminous bodies). Besides this, proto- 

 plasm contains globulin and albumin in small quantities, a 

 large proportion of water, a recognizable quantity of different 

 salts, and constantly changing products of metabolism, such as 

 fats, cholesterin, lecithin, glycogen, sugar, etc. Living proto- 

 plasm always has an alkaline reaction. 



Concerning the finer protoplasmic structure there are four 

 different and opposing views (Fig. 1). 



According to one view held at the present time by only a 



