2l6 HUMAN BIOLOGY 



orange background characterize the presence of malignant 

 islands within the normal tissue. Cells also modify their 

 medium by the production of larger or smaller amounts of 

 proteolytic ferment, growth-activating substances, etc. 

 A fragment of leucocytic film placed beside a pure culture 

 of fibroblasts causes an increase in the rate of proliferation 

 of the latter. This effect is due to the setting free of a growth- 

 activating substance in the medium. Certain malignant 

 fibroblasts attract wandering cells and receive from those 

 cells the substances which determine cell multiplication. 

 The substances may belong to the class of embryonic 

 proteins, such as are contained in very young cells. They 

 may also be protein split products. It is evident that tissues 

 possess, in some measure, the power to manufacture the 

 medium in which they live. 



The application of such a new method to a very old prob- 

 lem has brought to light some of the hitherto unsuspected 

 properties of living tissues which are instrumental in building 

 up unity from manifoldness. These properties until recently 

 have remained hidden because cells had always been studied 

 as independent units without consideration of their environ- 

 ment. It is imperative, on the other hand, to apprehend the 

 concrete event of a living cell and not merely the abstrac- 

 tions on which classical cytology is based. Tissue and blood 

 cells never escape the influence of their environment with- 

 out, as well as within, the organism. When removed from 

 the body of the embryonic or adult animal, they manifest 

 almost at once their latent potentialities. They are seen to 

 be endowed with attributes which compel them to respond 

 in a certain manner to given chemical substances. Even 

 when they have been separated from the organism for 

 several years, they keep elementary characteristics which 

 induce them to organize, despite the fact that there is no 

 organism to be formed. They are apparently endowed with 

 instincts which continue to manifest themselves, even when 

 they have become purposeless. 



The elements of the body, therefore, do not appear to be 

 integrated by a central principle. Ontogenic or regenerative 

 stimuli cannot be likened to the driving impulse of the 

 mind of a sculptor carving a statue. There is no need of an 



