150 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



conditions. These are shown in Figures 58-GO. Figure 58 is the only 

 cell of its kind which could be found. To describe it in terms of normal 

 mitosis, there are two chromatic masses with some interzonal filaments 

 extending in straight lines between them. Enclosing these straight 

 fibres is a sharply defined membrane (mb. nl.) slightly constricted near 

 the equatorial region. This membrane is less distinct in the neighbor- 

 hood of the chromatic masses. It could not be determined whether the 

 membrane enclosed the chromatin, or not. Outside the membrane 

 several fibres could be seen curving from pole to pole. Figure 59 is, 

 beyond all doubt, a later stage of the condition seen in Figure 58. The 

 cell outlines are sharply defined, and the equatorial membrane has 

 formed, dividing the cytoplasm. The membrane (mb. nl.) must be 

 identical with the one similarly designated in Figure 58. It can now 

 be seen to enclose the chromatin. It is deeply constricted at the 

 equator, and some indistinct fibres extend, within this membrane, 

 between the chromatic masses. Figure 60 is readily seen to be a stage 

 following that of Figure 59. The division of the cytoplasm is complete, 

 a clear space intervening between the two daughter cells. The mem- 

 brane, mb. nl., is very sharply constricted, and the two cells are still 

 united by it. The chromatin is in the form of large granules. Be- 

 tween the two chromatic masses extend some fibres which become 

 thicker and darker as they approach the chromatic masses. 



A careful search through the series failed to reveal any other stages 

 in this process of division, and nothing like it was seen in other worms. 

 It can hardly be doubted that the membrane, mb. nl., of Figure 60 

 is a nuclear membrane. The membrane, mb. nl., of Figure 58 is therefore 

 a nuclear membrane. Is it the old nuclear membrane, or a new one'? 

 The question cannot be answered with the evidence at hand. Early 

 stages of mitosis are present in abundance, but none showed any sign 

 of the persistence of the old nuclear membrane. However, divisions of 

 this type are so rare that we may readily suppose its earlier stages to 

 be absent in the preparation. If mb. nl. is the old nuclear membrane, 

 we have here a case of indirect cell-division, during which the nuclear 

 membrane persists and divides by constriction in amitotic fashion. In 

 some Protozoa the nuclear membrane normally persists during mitosis. 

 It has been maintained by some writers that, in ordinary mitosis, the 

 nuclear outline persists and can be made visible by certain methods 

 (Pfitzner, '83, '86; Waldeyer, '88). 



The other alternative is that mb. nl. is a neio nuclear membrane, 

 formed about both chromatic masses during the early telophase and 



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