1 8 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



begins to take the form of typical nuclei. The centriole detaches 

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

 remain either wholly intranuclear (Fig. 20, A, a, 22, A, a), or become 

 entirely extranuclear (Fig. 20, B, a, 22, B, a). 



Theory of Binudearity of Cells and Chromidia. In the infusoria, the 

 nuclear structure divides into two nuclei (Fig. 8); a large one, the 

 macronucleus or vegetative nucleus, which functions during the vegetative 

 life of the cell, and a small one lodged in a hollow 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- 

 n or kinetonucleus which controls the for- 

 mat i n f the flagellum. 



Starting from these facts, a few in- 

 B vestigators have tried to demonstrate 



Fig. 7 .-Chromidia in pro- that a11 Cells have two nuclei ' Recent 

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



uf^^^r^S. cytoplasm of most protozoa small chro- 

 maba histolytka. (After Hart- mophilic granules, like the chromatin 

 chromidia"' Nnclfuf ' chr ' material, which are supposed to emigrate 



from the nucleus during certain phases 



of development, and which are likened to the nuclear substance 

 (Fig. 7). These granules are called chromidia, and all the granules 

 scattered in the cytoplasm are designated as the chromidial structure 

 or chromidium. 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. 



CYTOPLASM.- -Appearance and Properties of Cytoplasm Cytoplasm 

 may be denned 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- 



