CELLS AND NUCLEI. 269 



their interior. Some nucleoli exhibit distinct amoeboid move- 

 ments ; such appear to me to be young nuclear nucleoli. 

 These nucleoli are possibly the nuclei of immigrant phago- 

 cytes, which have found their way into the large nuclei of 

 degenerating cells : compare the nuclei of leucocytes (Fig. 17), 

 which exhibit all the varieties of form ascribed by Carnoy to 

 nuclear nucleoli. In some cases at least this explanation is 

 undoubtedly correct. 



The Paranucleus (Ncbenkern}. It sometimes happens that a 

 more or less shrivelled, irregular mass of material is found out- 

 side a nucleus. This mass stains readily, and has been termed 

 the paranucleus ; such paranuclei are occasionally seen in 

 epithelial cells. The nature of the paranucleus is unknown, 

 but, in some cases at least, I believe I have traced its origin to 

 a degenerated nucleus. After division one nucleus remains, and 

 the other undergoes degeneration. I shall hereafter recur to 

 this subject. 



The Active Nucleus. Before and during the division of the 

 nucleus, a phenomenon which always precedes the division of 

 a cell, the nuclear fibre is seen to undergo remarkable changes 

 in its arrangement. These changes are spoken of as karyo- 

 mitosis, or karyokinesis. The student will find an account of 

 them in any modern text-book on histology. When these 

 changes occur, the division of the nucleus is said to be 

 indirect. 



The indirect process of nuclear division is of very wide, if 

 not universal, occurrence in both animals and plants. 



The old view was that a nucleus undergoes simple fission, a 

 change which is termed the direct mode of nuclear division. 

 Many observers have entirely denied its occurrence since the 

 discovery of indirect division. 



At one time I thought that the large nuclei of many insect 

 tissues would afford favourable objects for the study of indirect 

 cell-division, but, although the nuclear thread is well developed 

 in them, they unfortunately do not divide. And I have been 

 unable to observe indirect nuclear division in insects satisfac- 

 torily except in the early stages of spermatogenesis. Although 



