NUCLEUS 53 



living aggregates of material — stores of nutritive material, 

 or products of the cell's activity, either useful or useless. 

 The scattered thread-like or rod-like mitochondria and the 

 knot-like Golgi apparatus are more often supposed to form 

 part of the living protoplasm. The first-named at least 

 may be seen in living cells, but both are destroyed by the 

 usual methods of fixation, and require special demonstra- 

 tion. 



{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. 27). 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 

 between the living matter of the nucleus and that of the 

 cytoplasm. Cytoplasm and nucleoplasm form a " cell 

 firm," potent in their co-operation. In many cells it has 

 been show^n that fragments or extensions of the nucleus pass 

 into the cytoplasm, forming what is called a " chromidial 

 apparatus," which seems to be of much functional im- 

 portance. 



The nucleus often lies within a little nest in the midst 

 of the cell substance, but it may shift its position from one 

 part of the cell to another. It has a definite margin, but 

 this may be lost, e.g. before cell division begins. Inter- 

 nally, the living resting nucleus appears to be fluid and 

 quite homogeneous ; but if injured mechanically or 



