60 



IXTRODUCTION TO CYTOLOGY 



large numbers of nuclei are associated in one extensive mass of cyto- 

 plasm. Such a body is called a ccenocyte, and the coenocytic condition is 

 found in a number of the lower organisms. In the Uredinese (rusts) the 

 typical life history is made up of two phases, with uninucleate and 

 binucleate cells respectively. In certain Infusoria two kinds of nuclei 

 are regularly present. Thus in Par amcecium caudatum (Fig. 15) there is 

 one small micronucleus which divides by a peculiar form of mitosis, and 

 one large meganucleus (macronucleus) which divides amitotically. In 

 P. aurelia there are two micronuclei and one meganucleus, whereas in 



Fig. 14. 



V, portion of body of Vaucheria, showing 

 coenocytic condition; nuclei dark and 

 plastids in outline. C, portion of body of 

 Cladophora, showing semi-coenocytic 

 condition. 



Fig. 15. — Par amcecium caudatum un- 

 dergoing fission; mega- and micronuclei 

 dividing. {From Minchin, after Biitsehli 

 and Schewiakoff.) 



Stentor there may be one meganucleus and several micronuclei. In 

 general the meganucleus seems to be a storage organ of the cell; it may 

 disappear and be replaced by a new one. The micronucleus performs 

 the usual nuclear functions. The mammalian red blood corpuscle begins 

 its life as a nucleated cell, but later on the nucleus is lost. 



The position of the nucleus in the cell 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-vacuolated cell it ordinarily occupies the center of the cytoplasmic 

 mass, whereas 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. In the Cladophora ceae (Carter 1919) it is regularly imbedded, 



