ADDRESS BEFORE THE BRITISH ASSOCIATION. 57* 



consider free cell-formation in a blastema or exudation by an aggrega- 

 tion of molecules, in accordance with the views of Henle, as a common 

 phenomenon. This proposition was attacked with great energy by 

 Virchow in a series of memoirs published in his 'Archiv,' commencing 

 in Vol. 1, 1847, and finally received its death-blow in his published 

 lectures on Cellular Pathology, 1858. He maintained that in patho- 

 logical structures there was no instance of cell development de novo; 

 where a cell existed, there one must have been before. Cell-formation 

 was a continuous development by descent, which he formulated in the 

 expression omnis cellvla e celluld. 



KAKYOKINESIS. 



While the descent of cells from preexisting cells by division of the 

 nucleus during the development of the egg, in the embryos of plants 

 and animals, and in adult vegetable and animal tissues, both in healthy 

 and diseased conditions, had now become generally recognized, the 

 mechanism of the process by which the cleavage of the nucleus te^- 

 place was for a long time unknown. The discovery had to be deferred 

 until the optician had been able to construct lenses of a higher pene- 

 trative power, and the microscopist had learned the use of coloring 

 agents capable of dyeing the finest elements of the tissues. There was 

 reason to believe that in some cases a direct cleavage of the nucleus, to 

 be followed by a corresponding division of the cell into two parts, did 

 occur. In the period between 1870 and 1880 observations were made 

 by Schneider, Strasburger, Biitschli, Fol, van Beneden and Flemming, 

 which showed that the division of the nucleus and the cell was due to 

 a series of very remarkable changes, now known as indirect nuclear and 

 cell division, or karyokinesis. The changes within the nucleus are of 

 so complex a character that it is impossible to follow them in detail 

 without the use of appropriate illustrations. I shall have to content 

 myself, therefore, with an elementary sketch of the process. 



I have previously stated that the nucleus in its passive or resting 

 stage contains a very delicate network of threads or fibers. The first 

 stage in the process of nuclear division consists in the threads arrang- 

 ing themselves in loops and forming a compact coil within the nucleus. 

 The coil then becomes looser, the loops of threads shorten and thicken, 

 and somewhat later each looped thread splits longitudinally into two 

 portions. As the threads stain when coloring agents are applied to 

 them, they are called chromatin fibers, and the loose coil is the 

 chromosome (Waldeyer). 



As the process continues, the investing membrane of the nucleus 

 disappears, and the loops of threads arrange themselves within the 

 nucleus so that the closed ends of the loops are directed to a common 

 center, from which the loops radiate outwards and produce a starlike 



