42 THE ELEMENTS OF STRUCTURE. 



embryo with the "cambium" of plant cells; in 1846 Von Mohl em- 

 phasised the importance of the " protoplasm " in vegetable cells ; Ecker 

 (1849) compared the contractile substance of muscles with the living 

 matter of amoebae ; Bonders also referred the contractility from the wall 

 to the contents; Cohn suspected that the "sarcode" of animals and 

 the "protoplasm" of plants must be "in the highest degree analogous 

 substances ;" and finally, Max Schultze (1861), accepted the growing 

 belief that plants and animals were made of very similar living matter, 

 and defined the cell as a unit mass of nucleated protoplasm. 



Forms of Cells. The typical and primitive form of cell is 

 a sphere, a shape naturally assumed by a complex coherent 

 substance situated in a medium different from itself. Most 

 egg cells and many Protozoa retain this primitive form, but 

 the internal and external conditions of life (such as nutrition 

 and pressures) often evolve other shapes, oval, rectangular, 

 flattened, thread-like, stellate, and so on. 



Structure of Cells. In a cell we may distinguish : 



(a) The general cell substance or cytoplasm, which con- 

 sists partly of genuinely living stuff or protoplasm, and 

 partly of complex materials not really living ; 



(I}} A specialised kernel or nucleus, with a complex struc- 

 ture, and important, but hardly, as yet, definable functions ; 



(c) One or more specialised bodies called central 

 corpuscles or centrosomata which seem to be centres of 

 activity during cell division ; 



(d) A cell wall, which occurs in very varied form, or may 

 be entirely absent. 



(a) The Cell Substance. When a simple cell is examined 

 in its living state, it often appears approximately homo- 

 geneous. Its substance is usually slightly fluid, but it may 

 be firm and compact in passive cells. It is usually trans- 

 lucent, but there are often obscuring granules of different 

 kinds. 



In thinking of the cell substance or cytoplasm, we must 

 distinguish the genuinely living protoplasm, of whose nature 

 we know almost nothing, from associated substances, such 

 as proteids, carbohydrates, fats, pigments, &c., whose 

 chemical composition can be ascertained. The associated 

 substances which often crowd the protoplasm, are due to 

 the chemical ascent of food material towards protoplasm, and 

 to the chemical disruption which protoplasm undergoes or 

 produces as it lives. 



