CELLS. 43 



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) 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 may 

 distinguish the genuinely living protoplasm, of whose nature 

 we know almost nothing, from associated substances, such 

 as proteids, carbohydrates, fats, pigments, etc., whose 

 chemical composition can be ascertained. But it may be 

 that what we call protoplasm is a mixture of proteids and 

 other complex substances. 



(d) 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, but there is no trace of a 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. 20). 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, and it is generally believed that 

 there are complex actions and reactions between the living 

 matter of the nucleus and that of the cytoplasm. Perhaps 

 we may venture to say that cytoplasm and nucleoplasm form 

 a " cell firm," potent only in their mutual dependence. 



The nucleus often lies within a little nest in the midst of 



