CELLS. 45 
(4) 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 ; : 
(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 
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. 
(6) 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’ 
