ELEMENTS OF MICROBIAL CYTOLOGY 



17 



I' 





Fig. 4— Dif- 

 fuse nuclei of 

 bacteria. A, B. 

 mycoides. {After 

 GuUliermond.) 

 B, Thiothrix ten- 

 uis. {After 

 Swellengrebel.) 



It does not seem probable, therefore, that cells can exist without 

 their nuclei. Nevertheless, to the present time it has not been possible 

 to find conclusive proof of the presence of a true nucleus in bacteria. 

 The presence in their cells, however, of a great num- 

 ber of small chromatin grains like the chromatin ma- 

 terial of nuclei, and their evolution during the forma- 

 tion of spores, force the observer to admit that these 

 represent grains of nuclear substance, and that bac- 

 teria have a kind of diffuse nucleus, which is scattered 

 in the form of small grains (Fig. 4) in the cytoplasm 

 of the cell. 



Forms of Nuclei in Microorganisms. — ^The nucleus 

 of primitive microorganisms is far simpler than in 

 the higher forms, where it becomes fairly complex. 

 Consequently in the Cyanophycea or blue algse, the 

 lowest of all algse; the nucleus is in a very primitive state. It is 

 large, not separated from the cytoplasm by a membrane, and is made 

 up simply of a nuclear fluid and a chromatic network. The cyto- 

 plasm is confined to a thin cortical layer 

 and the nucleus nearly fiUs the cell (Fig. 5). 

 In other microorganisms the nucleus is 

 much more complex. Yet frequently this 

 nucleus is found in a primitive state quite 

 different from typical nuclei of higher 

 organisms. In some amoebse, the nucleus 

 is formed simply of a poorly defined mem- 

 brane filled with nuclear fluid, and a large 

 body of chromatin resembling a nucleolus 

 called the karyosome or centriole-nucleolus 

 (Fig. 21), because it acts both as a cen- 

 triole and as a nucleolus. In the center of 

 the karyosome is frequently seen a more 

 intensely chromophilic corpuscle corre- 

 sponding to the centriole (Fig. 20, B, a). 

 Many protozoa and some algae have a 

 centriole-nucleolus, but it is wholly enclosed in the nuclear fluid. 

 The chromatin appears as little grains or as a network (Fig. 20, A, a). 

 In the higher micro5rganisms (protozoa and fungi) the nucleus 



Fig. 5. — Nuclei of Cyano- 

 phycecB. A, Thread of Rivit- 

 laria buUaia with nuclei in 

 process of division. B,-D, 

 Fragments of threads of Calo- 

 thrix pidvinata shovring nuclear 

 division. 



