486 BIOLOGY OF THE PROTOZOA 



resulted in the formation of three perfect incHvidiials; one from the 

 transected anterior portion witliout a microniicleus, and two from 

 the normal division of the posterior, portion. One of the latter, 

 the more anterior part, although perfect is of minute size owing to 

 the fact that division of the cell takes place through the original 

 geometrical center, or the "division zone" of the cell. This minute 

 cell grows to normal size and ultimately divides although its division 

 is delayed. The original anterior fragment is perfect as far as 

 external appearances are concerned, but it has no micronucleus 

 and after seven or eight days it dies without dividing. 



This experiment, fully confirmed in the essential points by 

 Young (1922), indicates a progressive change in the protoplasm 

 in the inter-divisional period. Plxcept when a micronucleus is 

 present, young cells when cut are unable to regenerate the missing 

 parts. Fragments of old cells ha^'e the power to regenerate missing 

 parts even in the absence of a micronucleus. Such regeneration is 

 characteristic of cells in preparation for division and occurs with 

 every division. It follows therefore, that the formation of cirri 

 in these regeneration experiments is due to some condition of the 

 protoplasm in old cells which is not apparent in young ones and 

 illustrates one type of inter-divisional differentiation. 



These experiments also indicate another significant phenomenon 

 viz., the reorganization (de-dift'erentiation) of the protoplasm 

 with every division of the organism. When division is nearly 

 completed the power to regenerate without a micronucleus which 

 was possessed by the organism two hours before, is entirely lost 

 and fragments without a micronucleus remain as they were when 

 cut (Fig. 202). As stated above a young cell is unable to regenerate 

 unless the micronucleus is present and this possibility does not 

 appear in the protoplasm until after some hours of metabolic 

 activity. This strongly indicates the reorganization of the proto- 

 plasm or a restoration to a labile and undifferentiated condition. 

 Other evidences of de-differentiation are shown })y the loss through 

 absorption of the old memljranelles, cirri, undulating membranes, 

 oral baskets of the Chlamydodontidae and kinetic elements of dif- 

 ferent kinds (see Chapter V) while new elements replacing them 

 are developed from the protoplasm. In this way there is a more 

 or less complete reconstruction or reorganization of the organization 

 at each division. 



Another characteristic evidence of inter-divisional dilTerentiation 

 is shown by the polarization of the cell immefhately prior to divi- 

 sion whereby "division zones" are set up through which division of 

 the cell takes place. Such division zones first described by Popoff 

 (1907) are quite evident morphologicall\' in Frontonia leitcas and 

 physiologically in Paravieciwn caudatum or Uronychia transfiiga 

 (Fig. 20.'^). Paramecivm caiidatwi when cut near the anterior or 



