THE INTERPHASE NUCLEUS 



arises whether any corresponding visible events within the nucleus are 

 recognizable. Various modifications of mitosis are known in which the 

 ploidy of nuclei is changed (p 149). However, there is one example 

 where ploidy increases independently of any mitotic process. In the 

 ileum of the larval mosquito (Berger^"^), the nuclei grow in size, and 

 the chromosomes reduplicate without any visible changes within the 

 nuclei, which remain in the resting phase throughout. 



One example from a vertebrate tissue where, by a modified form of 

 mitosis, the degree of heteroploidy and nuclear size is doubled has 

 been described by Beams and King^^^ in their work on the regenerating 

 liver of the rat. In this animal two-thirds of the whole organ may be 

 removed experimentally, and the normal size will be regained within 

 a month. Extremely frequent mitoses are seen in the liver tissue within 

 a few days after the operation, and marked variation in size of the 

 interphase nuclei is then seen. Large polyploid nuclei are formed by 

 mitoses of binucleate cells (p 149), in which the chromosomes from 

 both nuclei join a common metaphase plate. Two polyploid daughter 

 nuclei are then formed in telophase. If the cell divides, two mono- 

 nucleate tetraploid daughter cells result; if not, the cell still remains 

 binucleate, but now the nuclei are tetraploid. By the repetition of 

 such mitoses octoploid nuclei will result. Beams and King studied the 

 distribution of nuclear size in mononucleate and binucleate cells in 

 both regenerating liver and in the normal pieces removed at the time 

 of operation. In the normal liver, as we have already discussed, this 

 distribution is bimodal (Figure 21) while it becomes markedly poly- 

 modal in the regenerating tissue. In the normal development of the 

 liver, tetraploid nuclei are presumably formed in the same way; the 

 commoner of the two modes of normal nuclear size is the large one, 

 which must correspond with the tetraploid grade. Sulkin^"^ is of 

 opinion that only 10 per cent of normal liver nuclei in the rat corres- 

 pond in volume to the diploid mode. In a careful study of the cytology 

 of this material Biesele^io finds that chromosome counts at metaphase 

 in both the normal and the regenerating organ indicate a proportion 

 of diploid nuclei of somewhere between one-half and two-thirds; four 

 to five per cent of octoploid nuclei are present in both. 



Variations in chromosomal size 



A further factor which aflfects the size of a nucleus is variation in the 

 size of the chromosomes themselves. Again, most of what is known on 

 this topic refers to the Diptera, particularly to the giant chromosomes 

 of the larval tissues. According to Hertwig (G.)2ii, the salivary gland 

 nucleus in Drosophila melanogaster increases in volume by at least 264 

 times during larval development. Similar chromosomes are present in 

 other larval organs as Balbiani^^^ mentions in his original description 



61 



