200 THE CELL 



proved subsequently to be right in the main, whilst at the same 

 time it has formed a good foundation for many further discoveries, 

 and has suggested a number of problems, which have not yet been 

 definitely solved. These problems may be expressed in a single 

 sentence : it was necessary, and to a certain extent is still 

 necessary, to follow more closely in every detail the movements 

 which, during nuclear division, and during the formation of the 

 characteristic figures, take place in the individual micro-chemical 

 particles of substance, which can be distinguished in the nucleus 

 and in the division figures ; that is to say, to trace the rearrange- 

 ments which occur in the nuclein granules, the linin framework, 

 the spindle fibrils, the centrosomes, and the nucleoli, etc. The 

 discovery of suitable objects for examination, such as the nuclei of 

 tissue cells of Salamander larvae (Flemming), and the eggs of 

 Ascaris megalocejphala (van Beneden), as well as the use of the 

 newer oil immersion and apochromatic lenses, and the improve- 

 ment in the manipulation of reagents and staining solutions, have 

 rendered progress in this direction possible. 



The greatest advance has at present been made in the investiga- 

 tion of the figures produced by the changes of place of the nuclein, 

 thanks in the main to the excellent experiments of Flemming 

 (VI. 12-17), and the supplementary investigations of van Beneden 

 (VI. 4),Rabl (VI. 53), Boveri (VI. 6), Strasburger (VI. 71-73), 

 and Guignard (VI. 23). 



Flemming, who has made his observations chiefly upon tissue 

 cells of Sahwiander larva?, distinguishes clearly between the 

 achromatin and chromatin portions of the nuclear figure, that is to 

 say, the unstainable spindle fibrils and plasmic radiations, and the 

 stainable nuclear loops, or segments, which rest upon their sur- 

 faces. He was the first to make the important discovery that 

 these latter split longitudinally. The explanation of these in- 

 teresting phenomena w T as afforded by the discoveries of Henser, 

 Guignard, van Beneden, and Rabl, w r ho all observed independently, 

 on different objects, that the halves of the divided segments 

 (chromosomes) separate, and move towards the nuclear poles, 

 forming the foundation for the daughter-nuclei. 



The changes of position of those substances, which are connected 

 with the development of I he spindle and the centrosomes, and 

 with the disappearance of the nucleoli, have been much less ac- 

 curately investigated. 



As concerns the spindle, very various opinions are held, both as 



