306 ALWIN M. PAPPENHEIMER 



OBSERVATIONS UPON LIVING CELLS 



In the study of the hving cells in vitro, certain difficulties 

 were encountered, and the chief of these was the uncertainty in 

 regard to the derivation of the growing elements. It was only 

 after the study arid comparison of a large number of different 

 preparations that certain types of cells could be recognized, 

 classified and their origin made reasonably certain. As has been 

 observed by all workers, the growing cells do not adhere to their 

 differentiated form, but assume a simpler and often indifferent 

 type. The recent work of Foot (9) upon the growth of bone- 

 marrow in vitro, ernphasizes the difficulty in identifying the 

 growing cells with definite constituents of the normal tissue. 



Even under approximately identical conditions, so far as they 

 can be analyzed, there is considerable variation in the extent of 

 the growth, and to a less degree, in its character. Thus of a 

 given series, only a certain proportion yields a definite growth, 

 others gradually degenerating. While such discrepancies are 

 undoubtedly due in large part to technical faults, it is rarely 

 possible to find the exact cause for the failure of growth. 



That the age of the frog from which the thymus is obtained 

 has some influence upon the growth, is suggested by the follow- 

 ing series. Two parallel sets of cultures were made, from half 

 of which the thymus was taken from a young frog (6 cm.), and 

 for the others, from a very large frog (9 cm.). The plasma of 

 the small frog was used for both series. Eighteen preparations 

 were made. Abundant growth was obtained in all but two of 

 the cultures of 'young' thymus, whereas only three of the cul- 

 tures of 'old' thymus grew and these but sparingly (33 per cent). 

 In another series, eight preparations of thymus from an old very 

 large frog, failed to show growth, while three controls of young 

 frog thymus in the same plasma, showed abundant growth. 

 There was unfortunately no opportunity to repeat these obser- 

 vations on a large scale, but the point deserves further study 

 because of its bearing upon the involution of the thymus in 

 adult life. It might be supposed that the plasma of old animals 

 would exert a deleterious influence upon the growth of the thymic 

 elements, but this has proven not to be the case. Some of the 



