THE NUCLEI OF UROLEPTUS MOBILIS 351 



lar nucleus, the beginning of themacronuclei, and a small nucleus 

 which degenerates. The chromosomes are eight in' number in 

 the first two divisions of the fertilization nucleus where they can 

 be counted without much difficulty. After the differentiating 

 second division there is a long pause, during which the young 

 macronucleus undergoes its metamorphosis. During this period, 

 which lasts for approximately 72 hours, the two functional mi- 

 cronuclei become more compact and smaller, and when they 

 finally divide, the spindles are so minute that no sure observation 

 can be made as to the number of chromosomes contained. In 

 mitoses from ordinary vegetative divisions, however, the number 

 is eight, the chromosomes frequently lying in four pairs similar 

 to the condition in the second maturation division but occa- 

 sionally one or more pairs are not united so that five, six, or 

 seven may be counted. 



h. Fate of the old macroriiicleus. All observers, with the ex- 

 ception of Prowazek ('99, '09), are agreed as to the fate of the 

 old macronucleus. Prowazek, among the recent workers at 

 least, appears to be alone in concluding that the old macronucleus 

 is cast out of the cell as waste material. His conclusion is drawn 

 from the premise that nucleins cannot be digested and must 

 therefore be eliminated. As Collin ('09) points out, Prowazek 

 describes the loss of staining power of the old macronucleus frag- 

 ments, and to this extent at least admits that the nuclei are di- 

 gested. As he offers no evidence of such elimination from direct 

 observation of the reorganization processes of Bursaria, Stylon- 

 ychia pustulata, and Lionotus fasciola, his conclusion may be 

 dismissed as highly improbable. 



The majority of observers have traced the gradual reduction in 

 size of the old macronucleus or its fragments until nothing re- 

 mains. This certainly is its history in Uroleptus mobilis, where 

 the eight old macronuclei become greatly reduced in size and then 

 fade away, one by one, until all are absorbed in the cytoplasm. 



The following table is a summary of the points made by dif- 

 ferent observers on different species of ciliates and is useful for 

 purposes of comparison. In most cases where two or more ob- 

 servers have worked on the same species only the most important 

 papers are cited. 



