HISTOLOGY AND CYTOLOGY 359 



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groups of cells. Pari passu with this gradual division of function 

 as we ascend the animal scale we find also a gradual differentia- 

 tion of structure whereby the cells originally equipotential, or 

 approximately so, and of similar structure, become totally unlike 

 and particularly fitted for different purposes. In the course of our 

 examination of the higher types we have noted numerous illustra- 

 tions of the differential specialisation of structure, and we have seen, 

 for example, how nerve cells, bone cells, glandular cells, muscles, and 

 so on, are quite different from one another, and each adapted to 

 perform its own work. However much they have been changed, 

 however, with few exceptions, they still remain cells, and we may 

 now consider the structure of an idealised cell from which they can 

 all be derived. Naturally this will be far more like the primitive 

 Protozoon than the cell of any tissue of a higher form with the 

 exception of the white blood corpuscles and the lymph cells, which 

 are not much specialised. 



The cell consists of a more or less independent mass of 

 protoplasm within which is a denser body, the nucleus, so that we 

 are able to distinguish a cell body composed of cytoplasm, and the 

 inner part the nucleus, the material of which is often termed nucleo- 

 plasm. Under the low 'powers of the microscope this protoplasm 

 appears as a granular, semi-transparent, greyish-white mass of very 

 viscous substance, and under higher magnification it is seen to present 

 a very characteristic reticulate appearance, which has been inter- 

 preted in different ways by various authorities. It was supposed 

 that protoplasm is composed of a sort of sponge-like interlacing 

 network of solid fibres, termed the spongioplasm, the interstices of 

 which are filled with a fluid substance, the cell sap or hyaloplasm. 

 There are certain objections from the physical aspect to this way of 

 looking at things, and while the terms have been retained here as 

 convenient for descriptive purposes, it is not intended to imply that 

 the theory with which they are associated is the correct one ; indeed, 

 it has been almost completely given up. The most generally accepted 

 explanation is that protoplasm possesses an emulsion or foam-like 

 structure resulting from the mixture of substances of different 

 surface tensions, and that the so-called fibres composing the spongio- 

 plasm are probably produced by the different optical properties of 

 the constituents, and that this appearance is retained permanently 

 when the protoplasm is killed. It is possible that actual condition 

 varies with the different stages of physiological activity of the cell. 

 When dealing with so small, delicate and transparent an object as 

 the living cell it is extremely difficult to make out its finer structure 

 accurately without staining it in some way to emphasise its com- 

 ponents. This procedure usually necessitates killing the cell in some 



