18 GENERAL CONSIDERATIONS 



with the micronucleus regenerates perfectly, but the amicronucleate frag- 

 ment, while it may live for a few days, never regenerates the missing 

 structures. The same result is obtained if the organism cut is from ten 

 to fifteen hours old. If, however, individuals older than this are cut in 

 the same manner, an increasing percentage of complete regenerations, 

 varying with increasing age, results (Fig. 2). At the time of the experi- 

 ment the interdivisional period was twenty-six hours. If individuals 

 were cut after twenty-one hours of age, regeneration of the amicronu- 

 cleate individuals was invariable. The experiment indicates a progres- 

 sive differentiation in respect, at least to the power to regenerate and, 

 to that extent, a change in organization. Furthermore, if one individual 

 is similarly cut while the two daughter cells at division are still con- 

 nected, or shortly afterwards, there is no regeneration of the amicronu- 

 cleate fragment. This indicates that the condition which underlies the 

 power to regenerate is lost with processes of division and is not regained 

 until the young cell has undergone a considerable period of normal 

 metabolism (Calkins, 1911). This experiment was confirmed by Young 

 (1922) (see also Summers, infra, Chapter XVI). 



Changes with Metabolism 



There is an accumulating amount of morphological evidence that a 

 derived structure, such as the macronucleus, is constantly changing with 

 continued metabolism. In Uroleptus halseyi reorganization after con- 

 jugation requires from four to five days for completion. At first there 

 is no chromatin in the young macronucleus, which stands out clearly 

 in the young organism, and attempts to stain it after fixation are futile. 

 By use of the Feulgen nucleal test, however, the chromatin reaction be- 

 comes increasingly intense and the chromatin granules more distinct 

 toward the end of the reorganization period; the nucleus disappears 

 from sight in the living organism, but now stains intensely with any 

 nuclear dye. The young macronucleus divides three times with the first 

 division of the ex-con jugant and each of the daughter cells receives 

 eight macronuclei, the chromatin staining deeply in all of them. This 

 chromatin is in the form of discrete granules of similar form and uni- 

 form size, but during the interdivisional period there appear a few 

 (three to five) larger granules, which are dissolved by the Feulgen 



