112 CICATRIZATION OF WOUNDS 



does not always succeed. It is not exactly child's play to 

 dissect the eye of an embryo in order to secure the pigmented 

 epithelium which surrounds the iris. 



Experiments- can be conducted only with cultures .at least 

 three months old. At that age they constitute relatively 

 homogeneous units. They can then be submitted to different 

 diets so as to permit the study of either their morphological 

 modifications, their physiological reactions, or alterations in 

 their general behaviour and mobility. 



These modifications can be reversible as, for instance, altera- 

 tions of the structures or dimensions of the cells and of their 

 elements, or irreversible, such as the transformation of one 

 type of cell into another. Certain cells develop only in groups 

 and form tissues (fibroblasts, epithelium, etc.). Others, on 

 the contrary, are independent, and manifest a certain individual 

 activity. Leucocytes (white cells) and macrophages are in this 

 category. The latter are found almost everywhere in the 

 organism. In blood, in lymph, in bone marrow, in conjunctive 

 tissue. When cultivated in plasma, macrophages transform 

 themselves into large cells surrounded by an undulating mem- 

 brane. The addition of certain substances such as amino- 

 peptones, organic enzymes, brings about the disappearance of 

 these membranes and the transformation of the mobile cells 

 into fiixed cells. But these changes are reversible. The mem- 

 branes reappear after a few days in an appropriate medium. 

 On the contrary, when the cells of a culture of macrophages 

 are transformed into fibroblasts by the addition of an extract 

 of Rous sarcoma (spontaneous chicken cancer) the change is 

 irreversible. The same is true of fibroblasts treated with 

 plasma containing heparin (heparin is a substance which 

 keeps plasma from coagulating). The fibroblasts turn into 

 macrophages, acquire all the physiological properties of the 

 latter — mobility and the power of phagocytosis — and conserve 

 them. 



A culture does not completely cease growing when it is 

 made to fast. Its life is, however, very much slowed down, as 

 was shown by Fischer. The conditions resemble much more 



