THE NUCLEUS 



centre of cell-activity, and hence a primary factor in growth, develop- 

 ment, and the transmission of specific qualities from cell to cell, and 

 so from one generation to another. 



i. General Structure 



The cell-nucleus passes through two widely different phases, one of 

 which is characteristic of cells in their ordinary or vegetative condi- 

 tion, while the other only occurs during the complicated changes 

 involved in cell-division. In the first phase, falsely characterized as 

 the "resting state," the nucleus usually appears as a rounded sac-like 

 body surrounded by a distinct membrane and containing a conspicu- 

 ous irregular network (Figs. 6, 7, 13), which is in some cases plainly 

 visible in the living cell (Fig. 9). The form of the nucleus, though 

 subject to variation, is on the whole singularly constant, and as a rule 

 shows no very definite relation to that of the cell-body, though in elon- 

 gated cells such as muscle-cells, in certain forms of parenchyma, 

 and in epithelial cells (Fig. 49), the nucleus is itself often elongated. 

 Typically spherical, it may, in certain cases, assume an irregular or 

 amoeboid form, may break up into a group of more or less completely 

 separated lobes (polymorphic nuclei, Fig. 49), sometimes forming an 

 irregular ring (" ring-nuclei " of leucocytes, giant-cells, etc., Fig. 14, D). 

 It is usually very large in gland-cells and others that show a very 

 active metabolism, and in such cases its surface is sometimes increased 

 by the formation of complex branches ramifying through the cell 

 (Fig. 14, E). 



These forms seem in general to be fairly constant in a given species 

 of cell, but in a large number of cases the nucleus has been seen in 

 the living cell (cartilage-cells, leucocytes, ova) to undergo more or less 

 active changes of form, sometimes so marked as to merit the name of 

 amoeboid (Fig. 77). Perhaps the most remarkable deviations from the 

 usual type of nucleus occur among the unicellular forms. In the cili- 

 ate Infusoria the nuclei are massive bodies of two kinds, viz. a large 

 macronucleus and one or more smaller micromtclei, both of which are 

 present in the same cell, the former kind being generally regarded as 

 the active nucleus, the latter as a reserve nucleus from which at cer- 

 tain periods new macronuclei arise (p. 224). The macronuclei show a 

 remarkable diversity of form and structure in different species. Still 

 more interesting are the so-called scattered or distributed nuclei, de- 

 scribed by Biitschli in flagellates and Bacteria, by Gruber in certain 

 rhizopods and Infusoria, and by several authors in the Cyanophycese 

 (Figs. 15, 1 6). The nuclear material is here apparently scattered 

 through the cell in the form of numerous minute, deeply stained gran- 

 ules, which, if this identification is correct, represent the most primi- 



