THE NUCLEUS. 43 



{b) The Nucleus. — Almost every cell contains a nucleus 

 or several. It used to be said that some very simple 

 animals which Hseckel called Monera had no nuclei, but in 

 several they have been recently discovered. 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. We may safely say that cells without nuclei are 

 very rare, though in some cells the nuclei are less differen- 

 tiated than in others. 



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. 3). 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. 



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, it is anything but homogeneous ; at any rate, homo- 

 geneous nuclei are rare. Usually there is a network of fine 

 strongly stainable (chromatin) strands, with less stainable 

 (achromatin) substance in the meshes. But in other cells, 

 or at another time in the same cell, the nucleus is seen to 

 contain a coiled (chromatin) thread, or a number of chro- 

 matin loops (Fig. 2). Weismann and others believe that 

 these chromatin elements or chromosomes are the bearers of 

 hypothetical bodies whose properties are supposed to deter- 

 mine the nature of an organism and its life. Many nuclei 

 also contain one or more little round bodies or nucleoli, 

 apparently of less importance. The term is applied some- 

 what vaguely to little aggregations of chromatin, and more 

 properly to vacuole-like bodies, in which some believe 

 that the waste products of the nucleus are collected. 



