64 THE CELL. 



C NUCLEAR AND CELL-DIVISION. 



The founders of the cell theory believed in what may be known 

 as a modification of the theory of spontaneous generation, stating that 

 cells might originate from a structureless substance known as kyto- 

 blastema or blastema, in which a nucleus was formed by precipita- 

 tion. Henle (1841) drew attention to the fact that cells might mul- 

 tiply by the separation of small portions of the cell-body, a process 

 known as budding ; and Barry (1841) stated that during the multi- 

 plication of cells the nuclei divided. The same year Remak 

 observed division of cells in the blood of embryos. Goodsir (1845) 

 originated the theory that all cells were developed from preexisting 

 cells. This was first clearly stated as a general law by Virchow 

 (1855), and his saying, " Omnis celLida a-cettula" is constantly being 

 verified. Our more accurate knowledge of cell-division dates, how- 

 ever, from more recent times (187380), when Schneider, Fol, Stras- 

 burger, Flemming, and many others demonstrated that during the 

 division of the cell the nucleus passed through a series of compli- 

 cated changes which resulted in an exact division of the chromatin. 



The phenomena which usher in cell-division are especially 

 /noticeable -in the nucleus, the elements of which are arranged and 

 transformed in a typic manner. During the division of the nucleus 

 the nuclear membrane is lost, and the relationship of the substances 

 of the nucleus to the protoplasm of the cell is a very intimate one. 

 As a consequence, during the middle phases of division there is no 

 well-defined demarcation between the nucleus and the cell-body. 

 As a rule, the mother cell and nucleus divide into two daughter 

 cells, each having a nucleus, alike in every particular. It was early 

 observed, however, that occasionally cells divided by a much sim- 

 pler process, in which case the nucleus did not pass through such 

 complicated changes. Accordingly, two distinct types of cell- 

 division are recognized, which are distinguished as mitosis, karyoki- 

 nesis, or indirect cell-division, and amitosis, or direct cell-division. 

 Both lead to the formation of two nuclei, which are known as 

 daughter nuclei as distinguished from the original mother nucleus. 



J. MITOSIS OR KARYOKINESIS (INDIRECT CELL-DIVISION). 



Th-e description of the process of mitotic cell-division is compli- 

 cated by the fact that structural changes are observed which occur 

 simultaneously in the nucleus, centrosome, and cytoplasm. This 

 fact should be borne in mind, as, for the sake of clearness, a sepa- 

 rate description of the changes involving each of these structures 

 seems demanded. The process of mitotic cell-division may be 

 divided into four periods or phases, which follow one another with- 

 out clearly defined limits : 



The prophascs, in which the nuclear membrane disappears, the 

 chromatin is transformed into definite threads, and the centrosome 



