REVERSIBILITY OF MORPHOGENESIS 15 



if food is withheld, a very active protein-splitting enzyme is soon 

 developed, whose activity leads to disintegration of the proto- 

 plasm and proteins of the cell with the appearance of amino 

 acids and other relatively simple substances. 



Protein decomposition and synthesis is a reversible process in 

 the sense in which the term reversibility is used in this paper 

 (defined above). Proof that living cells normally contain or can 

 develop a mechanism for a reversible process of cleavage and 

 synthesis of their own proteins is practically complete (Levene, 

 '05). 



From this viewpoint, dedifferentiation, such as described for 

 Bursaria and such as may be seen to a less striking degree in 

 many other ciliates should not be considered an exceptional 

 phenomenon, but rather one which occurs to a wide extent among 

 different types of cells. 



That dedifferentiation in Bursaria is distinct from the process 

 of breakdown and utilization of the cell substances for energy re- 

 quirements was shown by the fact that large individuals measur- 

 ing over 500 microns in length, when placed in tap .water and 

 starved for several days decreased in size until in many ob- 

 served cases the length of the individual was less than 90 microns. 

 Many of these starved individuals had fully differentiated and 

 normal gullets. In the starved individuals there is decrease in 

 size of the gullet but proportions are maintained. Dedifferen- 

 tiation may or may not occur during starvation. Often dif- 

 ferentiation of a dedifferentiated individual was seen to occur 

 after two or even three days of starvation in tap water when the 

 size of the individual was much decreased. These facts show 

 that dedifferentiation in Bursaria and probably in many other 

 kinds of cells is a rather distinct type of process, and that we 

 are not justified in considering it as identical with the ordinary 

 extensive autolytic changes in cells in vitro, or with katabolic proc- 

 esses which supply the energy requirements of the cell, but rather 

 as a related type of breakdown of colloidal complexes; this 

 breakdown being inhibited and controlled in a similar way to the 

 inhibition and control of the action of endoproteolytic enzymes. 

 According to experiments by Levene ('05) and more recently by 



