LIFE OUTSIDE OF ORGANISM CARREL AND BURROWS. 577 



be kept under the miscroscope for a long time, if necessary, without 

 any danger to the life of the tissue. Before the beginning of the 

 growth, the fragment of tissue appears as an opaque, sharply outlined 

 mass in the clear medium. In the surrounding clear medium the 

 growing cells are easily detected. Camera lucida drawings of the 

 cells can be made when the tissues develop slowly, like cartilage or 

 peritoneum. But even in these cases the motion of the cells and the 

 changes in their shape require that the sketches be made rapidly. 

 The growth of sarcoma or of spleen is often so rapid that it renders 

 impossible an accurate camera lucida drawing. The best method 

 of recording the morphology of the living cultures is to photograph 

 them. But this is often very difficult because the new tissue is dense 

 or the cells are faintly seen, and chiefly because the cells do not grow 

 on the same plane. Generally in a very actively growing culture no 

 cell can be seen distinctly. Even when the outlines of the cells can 

 be distinguished easily under the microscope a sharp photograph 

 of them may be impossible if they are surrounded by cells which 

 have grown on slightly different planes. 



For exact cytologic study the cultures are fixed and stained. The 

 cover glass, to which the culture is adherent, is separated from the 

 hollow slides, and immersed in corrosive sublimate, acetic acid, or 

 formalin, or the various preparations of potassium bichromate solu- 

 tions. Afterwards they are stained in hematoxylin. When the cul- 

 ture medium is spread on the cover glass in a very thin layer, and 

 when the culture is not too old, the cells appear very distinctly and 

 all their structural details are easily observed. When the plasmatic 

 medium is thick, and when the cells have grown in many different 

 planes, serial sections of the hardened culture are required. 



In order to increase the length of a primary culture secondary and 

 tertiary cultures are made. A secondary culture is obtained from a 

 primary culture by extirpation with a fine needle of the fragment 

 of original tissue, which is deposited on a cover glass and covered 

 with fresh plasma. A second generation of cells can be obtained 

 from a primary or secondary culture by two procedures, one consists 

 of extirpating the fragment of tissue from the culture medium and 

 of covering the free space with fresh plasma. Then the cells from 

 the old plasma grow into the new plasma. The other consists of 

 cutting with fine scissors fragments of medium containing the cells 

 from which a second generation must be obtained. The fragment is 

 deposited on a cover glass and covered with plasma. 



The cultures can be grafted under the skin of an animal. Small 

 cultures, or large cultures on plate are used. A fragment of the 

 medium containing the living cells is cut with a knife and introduced 

 into the tissues of the animal. 

 9757S°— sm 1910 37 



