VITALITY 263 



In such cases the anterior portion may live for four or five days 

 as an amorphous fragment, but it never regenerates the giant cirri. 

 The posterior part, however, regenerates the missing anterior region 

 within a few hours and becomes a perfect cell. Exactly the same 

 result invariably follows if an individual is cut when five to eight or 

 ten hours old after division (Fig. 135, 3). At this time the normal 

 individual is fully grown and active. At the age of sixteen to eigh- 

 teen hours different results are obtained. If a number of individuals 

 are cut at this age a small percentage of the anterior parts without 

 micronuclei will regenerate into perfect individuals save for absence 

 of the micronuclei; the posterior parts always regenerate. This 

 percentage rises to 100 per cent of cases when individuals twenty- 

 four hours' old are cut. Under the conditions at the time the 

 experiments were made divisions occurred in normal animals at 

 intervals of twenty-six hours. Older cells, when cut, frequently 

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

 transected anterior portion without a micronucleus 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 divi- 

 sion 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. Except when a micronucleus is present, 

 young cells when cut are unable to regenerate the missing parts. 

 Fragments of old cells have the power to regenerate missing parts 

 even in the absence of a micronucleus. Such regeneration is char- 

 acteristic 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-differentiation) of the protoplasm with 

 every division of the organism, a phenomenon fully confirmed by 

 Taylor (1928). When division is nearly completed the power to 

 regenerate without a micronucleus which was possessed by the or- 

 ganism two hours before is entirely lost and fragments without a 

 micronucleus remain as they were when cut (Fig. 135). 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 



