DIRECT DIVISION 13 



AMITOSIS. 



The simplest form of cell division is one which rarely occurs. Ordina- 

 rily the division of the cell is accompanied with the production of proto- 

 plasmic filaments, and the process is therefore called mitosis (Greek, JU/TOS, 

 a thread). But in direct division or amitosis these filaments are not 

 developed. The nucleus merely becomes increasingly constricted at 

 the middle until divided in two; or it may be bisected by a deep cleft or 

 fissure. Preceding the division of the nucleus, the nucleolus, if present, 

 may subdivide and supply each half of the nucleus with a nucleolus (Fig. 

 u). Cells which divide by this method are usually degenerating, and 



j 



Beginning Completed Beginning Completed 



Division of the nucleolus. Division of the nucleus. 



FIG. n. AMITOSIS IN EPITHELIAL CELLS FROM THE BLADDER OF A MOUSE. Xs6o. 

 Such preparations as that shown in the figure are made by pressing the lining of a freshly obtained 

 piece of the bladder against a clean cover-glass. Certain of the superficial cells adhere to it, and they are 

 then fixed and stained. 



the process may terminate with the multiplication of nuclei. If carried 

 to completion, the protoplasm also divides, and a cell membrane develops 

 between the daughter nuclei. The r61e of the centrosome in amitosis 

 has not been determined. Maximow finds it in a passive condition 

 between the two halves of the nucleus, or beside the stalk connecting 

 these halves if the division is not complete (Anat. Anz., 1908, vol. 33, p. 

 89). He states that certain mesenchymal cells which divide by amitosis 

 in the rabbit embryo are not degenerating, but may later divide by mito- 

 sis, and thus he confirms Patterson's similar conclusion in regard to cer- 

 tain cells in the pigeon's egg. These instances are regarded as exceptional. 

 In the human body the detachment of a portion of the lobate nucleus 

 of certain leucocytes has been described as amitotic division, but the 

 superficial cells of the bladder furnish more typical examples. E. F. 

 Clark has found many cells dividing by amitosis in the degenerating parts 

 of a human cancer. The occurrence of two nuclei within one cell by no 

 means indicates this form of division. Associated with such cells, others 

 containing nuclei of the dumb-bell shape, or those partially bisected 

 by clefts must be found, in order to prove that amitotic division is taking 

 place. 



