CELLS. 45 



(b) A specialised nucleus, with a complex structure, 

 and important functions ; 



(c) One or more specialised bodies called central 

 corpuscles or centrosomes, which seem to be centres of 

 activity during cell division ; 



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

 be entirely absent. 



(a) As to the cell substance, it often appears at first sight 

 almost homogeneous, but higher magnification shows con- 

 siderable structural complexity. It is certainly not like 

 white of egg, but shows a reticular, fibrillar, or vacuolar 

 structure. It is usually slightly fluid, but it may be firm 

 and compact in passive cells. It is usually translucent, but 

 there are often obscuring granules of different kinds. 



In thinking of the cell substance or cytoplasm, we 

 distinguish the genuinely living protoplasm, which may be 

 a mixture of proteids, from other materials of simpler 

 chemical composition, such as carbohydrates, fats, pigments, 

 etc. Some of these may be nutritive materials in process 

 of elaboration into more complex substances; others are 

 disruptive products of the metabolism. 



(b) As to the nucleus, one at least is present in almost 

 every cell. It used to be said that some very simple 

 animals, which Haeckel called Monera, had no nuclei, but in 

 many cases the nuclei have now been demonstrated. In 

 other cases, e.g. some Infusorians, the nuclear material seems 

 to be diffused in the cell substance. The red blood cells 

 of Mammals seem to be distinctly nucleated in their early 

 stages, though there is no nucleus in those which are full 

 grown. 



The nucleus is a very important part of the cell, but it is 

 not yet possible to define precisely what its importance is. 

 In fertilisation an essential process is the union of the 

 nucleus of the spermatozoon or male cell with the nucleus 

 of the ovum or female cell (Fig. 23). In cell division the 

 nucleus certainly plays an essential part. Cells bereft of 

 their nuclei die, or live for a while a crippled life. Accord- 

 ing to some, the nucleus is important in connection with 

 the nutrition of the cell ; according to others, it is of special 

 importance in connection with the respiration of the cell. 

 It is certain that there are complex actions and reactions 



