42 THE ELEMENTS OF STRUCTURE. 



several. It used to be said that those 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. But we 

 may safely say that cells without nuclei are very rare, though 

 in some cells the nuclei are certainly less differentiated 

 than in others. 



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

 not possible to define precisely what its importance is. In 

 fertilisation, the most essential process is the union of the 

 nucleus of the spermatozoon or male-cell with the nucleus 

 of the ovum or female cell. In cell-division, the nucleus 

 certainly plays an essential part. Cells bereft of their 

 nuclei die, or for a time live a crippled life. On the other 

 hand, there is no doubt that the substance of the cell also 

 influences the nucleus. 



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

 the cell-substance, but it may shift its position within the 

 cell. It has a definite margin, but this may be lost, e.g., 

 before cell-division begins. Internally, it is anything but 

 homogeneous ; at any rate, homogeneous nuclei are rare. 

 As in the general cell-substance, so here, the histologists 

 have discovered a network of fine strongly stainable (chro- 

 matin) 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 chromatin loops. Many nuclei also 

 contain one or more little round bodies or nucleoli, appar- 

 ently of less importance. Within the nucleolus an " endo- 

 nucleolus" has been discovered. Though the nuclei of 

 different cells are often different in small details, there is a 

 marvellous unity, both of structure and activity, throughout 

 the world of cells. 



{c) The Cell- Wall — To the earlier histologists, who often 

 spoke of cells as little bags or boxes, the wall seemed of 

 much moment. It is, however, the least important part of the 

 cell. In plant cells there is usually a very distinct wall con- 

 sisting of cellulose. This has the same chemical composition 



