l8 MORPHOLOGY AND CULTURE OP MICROORGANISMS 



biegins to take the form of t3T)ical nuclei. The centriole detaches 

 itself from the karyosome which becomes a true nucleolus, and may 

 remain either wholly intranuclear (Fig. ig, A, a, 21, A, a), ot become 

 entirely extranuclear (Fig. 19, B, a, 21, B, a). 



Theory of Binuclearity of Cells and Chromidia. — In the infusoria, the 

 nuclear structure divides into two nuclei (Fig; 7); a large one, the 

 macronucleus or vegetative nucleus, which functions during the vegetative 

 life of the cell, and a small one lodged in a hoUow of the macronucleus, 

 the reproductive nucleus or micronucleus. At fertilization, the macro- 

 nucleus is disorganized and its place taken by the micronucleus which 

 reproduces by division both a micronucleus and a macronucleus. 

 Certain flagellates have likewise two nuclei, a large vegetative and re- 

 productive nucleus, and a small micro- 

 ox kinetonucleus which controls the for- 

 mation of the flageUum. 



Starting from these facts, a few in- 

 vestigators have tried to demonstrate 



Fig. 6.-Chronudia in pro- ^^^ ^^^ '^^'^^ ^^^^ ^^O ^^'^^^^^ ^^""^"^^ 

 tozoa. A, The cycle of the mi- evidence reveals that there are in the 



STri^wS^^B.'^K ^yt°Pl^^^ °f "^°«t protozoa smaU chro- 

 mcBba hystolytica. {After Bart- mophilic granules, like the chromatin 

 diromdia!' ^"'^^'^®' *^^''- material, which are supposed to emigrate 



from the nucleus during certain phases 

 of development, and which are Ukened to the nuclear substance 

 (Fig. 6). These granules are called chromidia, and all the granules 

 scattered in the cytoplasm are designated as the chromidial _^siructure 

 or chromidi^m. Chromidia have been found in the cells of higher 

 organisms. There is a theory that this chromidial system repre- 

 sents a second nucleus, the vegetative nucleus, scattered in the cyto- 

 plasm^ and that the entire cell is provided with two nuclei, one of 

 which has passed unseen up to this time because of its diffuse form. 

 This theory is much doubted to-day, and it seems probable that the 

 chromidium is simply a reserve material for the cell, or corresponds 

 to formations which will be described later as mitochondria. 



Cytoflasu.— Appearance and Properties of Cytoplasm.— CytoTpla.sm 

 may be defined for our purposes as a semi-fluid substance, granular in 

 appearance, and reacting with an acid stain. It has three essential 

 physiological properties, nutrition, motility, and sensibility. Cyto- 



