2 E. J. LUND 



cells when by division or otherwise, they replace lost cells which 

 may have had a different structure and function? Is there a 

 direct transformation of one type of cell or differentiated part 

 of a cell into another, and if so how is this transformation brought 

 about? What is the nature of the physico-chemical changes 

 which constitute structural regulation in the cell? 



The following facts derived from a careful study of over 3500 

 individuals of the ciliate protozoan Bursaria sp. are presented 

 with the hope that they may help to throw light on these ques- 

 tions and possibly point out what is the general nature of the 

 process which brings about dedifferentiation. 



II. THE NORMAL PROCESS OF DIFFERENTIATION AND 

 DEDIFFERENTIATION 



1. During cell division 



In any structurally differentiated cell which does not possess 

 any plane of symmetry it is necessarily true that cell division 

 would result in the formation of dissimilar parts unless some proc- 

 ess took place through which the daughter cells became alike. 

 The more striking cases of this kind occur among the more dif- 

 ferentiated types of protozoa Wallengren ('01). But even in 

 this group it is difficult to find examples where the process of 

 structural regulation at division can he followed with any degree 

 of ease. Bursaria, a heterotrichous ciliate of fairly common oc- 

 currence, is of sufficient size and complexity of organization to 

 permit one to follow with relative clearness the change in struc- 

 ture which occurs during cell division, regeneration and certain 

 othes processes. 



Figures 1 to 6 inclusive, of plate I, show ventral views of Bur- 

 saria in different stages of division. 1 The closure of the gullet 

 and peristome results in a complete disappearance of the mem- 

 braneDes lining the dorsal wall of the gullet (figs. 2 and 3, m), 

 and other structures such as the ciliated ridge or septum (fig. 1 , s) . 

 The changes may be better seen in cross sections from a plane 



1 I wish to express my indebtedness to Miss Helen Sanborn for the drawing 

 of all the figures except those in plate 4. 



