THE NUCLEUS 49 



nuclei produced by repeated division, as in the Siphonales and Phycomy- 

 cetes. In such a body a few partitions may be formed, giving cells with 

 varying numbers of nuclei, as in Cladophora (Fig. 4, C). Again, the 

 subdivision into cells may be very closely correlated with the division of 

 the nuclei so that every cell has one nucleus; this is the condition in the 

 majority of organisms. Frequently, cells with two or more nuclei occur 

 regularly in certain tissues of plants with uninucleate cells elsewhere 

 throughout the body, as in the internodal cells of Characese, tapetal cells, 

 and a number of other instances in vascular plants.^ A peculiar con- 

 dition is found in the red blood corpuscles of mammals. These cells 

 are originally nucleated, and the later enucleate condition is usually 

 attributed to an actual loss or degeneration of the nucleus. In amphib- 

 ians the enucleate erythrocyte ("erythroplastid") arises by the division of 

 a nucleated cell without an accompanying nuclear division.^ Enucleate 

 leucocytic elements are also known. The nuclei of a protoplast, whether 

 many or one, are called the nucleome by Dangeard. 



In certain infusoria two unlike nuclei are present: a small micro- 

 nucleus and a much larger macronucleus (Fig. 5). This is usually the 

 condition in Paramoecmm caudatum, but races with no micronucleus are 

 known."* Some species may have more than one micronucleus: in P. 

 woodruffi there are usually three or four, although the number varies 

 from none to eight (Wenrich, 1928a). The micronucleus divides mitoti- 

 cally and the macronucleus is usually said to divide amitotically, but at 

 least in some cases the story is evidently less simple than this (Mac- 

 Dougall, 1925). The macronucleus appears to be primarily a special 

 nutritive organ, showing decided alterations under changing environ- 

 mental conditions (Stolte, 1922). At certain intervals it is absorbed 

 completely and replaced by a new one derived by a process of division 

 from the micronucleus. C. V. Taylor (1923a), working with Euplotes, 

 observed that individuals from which the micronuclei had been arti- 

 ficially removed underwent only one or two more fissions and died in a 

 few days. 



The position of the nucleus is determined largely by physical causes, 

 such as surface tension, the position of the vacuoles, and the relative 

 density of the cytoplasm in different portions of the cell. In a non- 

 vacuolate cell it ordinarily occupies the center of the cytoplasmic mass. 

 In a cell with vacuoles it is imbedded in the cytoplasm even when the 

 latter is reduced to a thin parietal layer; it never lies free in the vacuole. 

 Its position is also related to the functions of the cell ; generally speaking, 



- Arber (1920). A historical review of known cases is given by Beer and Arber 

 (1920). See also Tischler (1921-1922, pp. 212if.) 

 3 Beyer (1921), Emmel (1924, 1925). 

 ^Landis (1920), Woodruflf (1921). 



