y ///; NUCLEUS 23 



cell, and through this is especially concerned with the formative proc- 

 esses involved in growth and development. For these and many 

 other reasons, to be discussed hereafter, the nucleus is generally re- 

 garded as a controlling centre of cell-activity, and hence a primary 

 factor in growth, development, and the transmission of specific quali 

 ties 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 con- 

 dition, 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 

 conspicuous irregular network (Figs. 5, 7, 10). Its form, though 

 subject to variation, is on the whole singularly constant, and shows 

 no definite relation to that of the cell in which it lies. Typically 

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

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

 rated lobes (polymorphic nuclei), or may be perforated to form an 

 irregular ring (Fig. n, 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. n, E). Interesting modi- 

 fications of the nucleus occur in the unicellular forms. In the 

 ciliate Infusoria the body contains nuclei of two kinds, viz. a large 

 macro-nucleus and one or more smaller micronuclei. The first of 

 these shows a remarkable diversity of structure in different forms, 

 being often greatly elongated and sometimes showing a moniliform 

 structure like a string of beads. In TracJielocerca and some other 

 Infusoria, according to Gruber ('84), the nucleus is not a single definite 

 body, but is represented by minute granules scattered throughout the 

 cell-substance (Fig. 12) ; Butschli describes somewhat similar diffused 

 nuclei in some of the Flagellates, and in the Bacteria. 



In the ordinary forms of nuclei in their resting state the following 

 structural elements may as a rule be distinguished (Figs. 5,6,7, 10, n): 



a. The nuclear membrane, a well-defined delicate wall which gives 

 the nucleus a sharp contour and differentiates it clearly from the 

 surrounding cytoplasm. 



b. The nuclear rcticulum. This, the most essential part of the 

 nucleus, forms an irregular branching network or rcticulum which 

 consists of two very different constituents. The first of these, the 



