74 



THE PROTOZOA 



Every transition from the one type of structure to the other may 

 be found in the nuclei of Protozoa ; in a vesicular nucleus the prin- 

 cipal mass of chromatin may break up into smaller grains which 

 become distributed throughout the nuclear cavity ; in a granular 

 nucleus some or all of the grains of chromatin may be clumped 

 together, and become fused to form a principal or single mass of 

 chromatin. Such changes may take place during successive periods 

 of activity of one and the same nucleus (Fig. 34). It is usual to 

 speak of the condition of the nucleus as " resting " when it is not 

 actually undergoing the process of reproduction by division ; but 

 it must be borne in mind that, so long as the cell is in a state of 

 physiological activity of any kind, the nucleus also shares in this 

 activity, and, strictly speaking, cannot be said to be resting. The 

 activity of the nucleus is expressed in continual changes in its 

 structure and rearrangements of its chromatin-substance and other 

 constituents. In the gregarine Porospora gigantea, Leger and 

 Duboscq (72) have observed changes taking place rhythmically in 



\ 



FIG. 35. Successive stages of the karyosome (see p. 76) of Porospora gigantea, 

 showing the transformation of a hollow into a homogeneous karyosoine by 

 expulsion of a vacuole of clear viscous fluid into the nuclear cavity, where it 

 forms a little mass of chromatin in front of the rnicropyle. After Leger and 

 Duboscq (72). 



the living condition (Fig. 35) ; compare also Chagas (48'5). Hert- 

 wig (64) has shown that the structure of the nucleus of Actino- 

 sphcerium can be correlated with the functional activities of the 

 cell. Thus a condition with the chromatin all concentrated to 

 form a central endosorne is found prior to division of the nucleus, 

 and is also found when the animal is being starved ; on the other 

 hand, when it is supplied with abundant nutriment and is feeding 

 actively, the chromatin-grains spread over the whole nuclear space. 

 Since, however, abundant food also leads to frequent nuclear 

 division, the condition with the chromatin concentrated at the 

 centre also occurs during active cell-metabolism, as well as during 

 hunger-periods . 



In the simplest condition of the nucleus the grain or grains of 

 chromatin are lodged in a space or vacuole, containing a clear fluid 

 or nuclear sap, but not enclosed by a definite membrane. Nuclei 

 of this simple type of structure are seen in some of the primitive 

 forms, such as the small amoebae of the Umax-type, in which the 



