AMITOSIS IN CELLS GROWING IN VITRO. 447 



as possible, and the light immediately turned off. The diffi- 

 culties attending direct and prolonged observation of the living 

 cell are not inconsiderable, for the eye must become accustomed 

 to distinguish minute structures through the contrast afforded 

 by varying grades of refractivity. At first only the most re- 

 fractive bodies are discernible, standing out as bright points or 

 lines, but gradually the less obvious structures, as mitochondria, 

 and amoeboid cell processes, come into view. Migration, in 

 many of the cells, is quite active, and hence it is necessary to 

 make observations frequently to prevent the cell from wandering 

 away from the field of vision and becoming lost. The extreme 

 sensitivity of the cell to light and heat, and to changes in osmotic 

 pressure of the media from evaporation within the moist chamber, 

 is responsible for the untimely termination of many observations, 

 and this difficulty becomes the more important when it is realized 

 that the study of a single cell must cover many hours to be com- 

 plete. 



It was at first planned to select a single living normal cell and 

 observe it at frequent intervals over a long period in the hope 

 that eventually a cell would be found which would divide by 

 amitosis. This course, however, did not prove to be practicable 

 on account of the infrequency with which amitosis, even of the 

 nucleus alone, is met with. In a series of 20 fixed and stained 

 growths from the heart of the embryo chick, in which there w r as 

 a total of 41,725 cells, 1 only 50 constricted nuclei, which could be 

 regarded as directly dividing, were found (a ratio of I to 835), 

 and thus the chances of the occurrence of amitotic nuclear divi- 

 sion in a cell selected at random were but little better than one 

 tenth of one per cent. To avoid loss of time, therefore, the plan 

 was adopted of selecting a cell in which the amitotic process 



1 The method adopted in making counts was as follows: 20 good preparations 

 from cultures of chick heart of various ages and stages of growth which had been 

 fixed and stained were selected. A small square was ruled with a diamond upon a 

 piece of glass after the method of Isaacs ('15), and this ruled glass was inserted in 

 the ocular so that a definitely outlined field was marked off upon the tissue culture 

 preparation on the stage. By manipulating the mechanical stage successive 

 fields could easily be brought into view, and the cells contained in them counted; 

 in this way all the cells in the entire new growth were counted except imperfect 

 cells and those near the original piece, which were several layers deep and were 

 1 ndistinct. 



