REVERSIBILITY OF MORPHOGENESIS 7 



wise is normally differentiated. This abnormal part may re- 

 main as such for many hours unless dedifferentiation leading to 

 redifferentiation or encystment takes place. If the injury to 

 the anlage is greater, dedifferentiation occurs, so as to eliminate 

 all visible traces of the anlage, with subsequent differentiation to 

 fully normal form. From observations on the numerous indi- 

 viduals injured in this way, it seems that considerable variation 

 occurs as to how great the mechanical injury to the anlage 

 must be before dedifferentiation sets in. The experiment is 

 most easily carried out on a dividing individual. From this it 

 appears that dedifferentiation proceeds only to the extent neces- 

 sary for complete duplication of the structure. One might 

 crudely compare the process to the rebuilding of a damaged 

 house; if the old foundation is uninjured or only slightly injured 

 it may be used again for supporting a new frame; while if the 

 foundation is seriously injured it also must be rebuilt. We see 

 here the essentials of the phenomenon of localization as found in 

 the development of the egg of many different metazoa, for from 

 the above experiments there appears to exist in the anlage of the 

 gullet a definite part which corresponds to a definite structure 

 in the fully differentiated gullet. 



IV. DEDIFFERENTIATION AND DIFFERENTIATION IN 

 HETEROMORPHS 



Throughout the work a total of about seventy-five abnormal 

 individuals was observed. A few of these were studied care- 

 fully, and observations made from time to time in order to follow 

 their histories. Very close observations on such individuals 

 were difficult to make because of continuous movements and the 

 impossibility of manipulating them for observation under higher 

 powers. This nearly always resulted in loss or injury to the 

 specimen whose history was desired. The observations were 

 made under low and high powers of the Leitz binocular and also 

 with the low power of the compound microscope Drawings 

 were necessarily made free hand, but care was taken to obtain 

 approximately correct proportions. Some of the individuals 



