V. 



Cell-division. 



ALTHOUGH the main general features of cell-division are familiar 

 to most readers of Natural Science, yet it is probable that 

 not a few have been unable to find time to keep pace with even a part 

 of the ever-increasing literature on the subject. So many varied, 

 and in some cases contradicting, statements have been made recently, 

 as to the real nature of the processes of cell-division, that any attempt 

 to summarise them as probable truths is necessarily accompanied 

 with considerable difficulty, and a likelihood to prove unintelligible 

 or misleading. Nevertheless, it is hoped in this article to give some 

 account of the chief features of ordinary cell-division looked at in 

 the light of the latest researches. Ever since Strasburger, Biitschli, 

 and Fol in 1870 drew attention to the extraordinary changes which 

 generally take place in the nucleus of a dividing cell, a large army 

 of investigators have given their almost undivided attention to 

 increasing our knowledge of the subject, and it is a matter of no 

 small regret that in such a wide field of research, which affords so 

 many chances of gaining deeper insight into some of the most striking 

 phenomena exhibited by living matter, few, if any, English names 

 stand pre-eminent. 



A cell may divide either (a) mitotically, that is to say, may pass 

 through a process of change known as mitosis or karyokinesis, 

 whereby a very perfect division of the chromatin, i.e., the deeply- 

 staining portion, in the nucleus, is brought about, or (b) ami- 

 totically, where a rough-and-ready splitting of the nucleus is 

 substituted for the more elaborate mitotic division. The former 

 is by far the more usual of the two, and although there are 

 undoubted cases w^here amitotic division is the rule, it is probable 

 that karyokinesis is even of wider occurrence than it is thought by 

 many to be. It is with this form of nuclear division that the present 

 contribution exclusively deals, and it will be found convenient to 

 study the subject from the following points of view, viz. : — 



(i.) The general features of karyokinesis. 



(2.) The origin, nature, and function of the component parts of 

 the nuclear figure. 



(3.) The relation of karyokinesis to fertilisation and segmen- 

 tation. 



