814 THE POPULAR SCIENCE MONTHLY. 



torial plate is formed, inclosing a narrow equatorial plane. This is the 

 plane of cell-division. A furrow appears around the equator of the 

 cell, which deepens, and extends inward between the equatorial plates. 

 It continues to deepen until it finally meets in the center, and the cell 

 is separated into two new ones. 



"While this is proceeding, new nuclei are forming at the nuclear 

 poles. The fibers of the daughter-stars pass through a series of changes 

 opposite to those above described. The ends of the loops unite until 

 a wreath is formed. This wreath soon becomes an irregular convolu- 

 tion, which quickly assumes the reticular structure. Membranes form 

 around the new nuclei. Nucleoli and granules reappear. The resting- 

 stage is regained. The original cell is replaced by two daughter- 

 cells. 



The above description, with its detailed account of the process of 

 cell-division, is not accepted in all its particulars by other observers. 

 There is great diversity of opinion about many points, which can not 

 be settled without much further investigation. It is also very prob- 

 able that much of the diversity of opinion arises from the fact that 

 the cells of different organisms vary in their features of change, and 

 that vegetable cells only distantly resemble animal cells in this par- 

 ticular. 



Some of the unsettled questions are the following : Klein and 

 Strasburger see little importance in the nucleolus. Klein doubts its 

 existence. There is an open question whether it and the granules are 

 not merely the nodes of the network. But the majority of observers 

 speak of the nucleoli and granules as lying free in the ground-sub- 

 stance, in the intervals of the network. It is also a question whether 

 or not the outer cell-substance is like the nucleus in structure. Klein 

 holds that it is. Flemming has lately announced the discovery, in the 

 resting-nucleus, of a very fine network, in connection with the coarser 

 one already known. He also declares that the membrane surrounding 

 the nucleus is really composed of minute flat plates of chromatin con- 

 tinuous. with, the fibrils of the network. These are separated by slight 

 intervals, so that the membrane seems pierced by holes, which per- 

 haps may be occupied by the transparent ground-substance. Others 

 deny the existence of a nuclear membrane, and think that it is an op- 

 tical illusion, caused by the arrangement of the fibers. Dr. Pfitzner 

 has recently declared that the chromatin fibrils are not homogeneous 

 in structure, but that they really consist of minute spherules of chro- 

 matin, held together by some other substance, probably achromatin. 



Such are some of the questions to be yet settled. It would appear 

 that the chromatin of the original nucleus becomes first regularly ar- 

 ranged around its center, then divides equatorially and recedes to its 

 poles, where it forms new nuclei, while the achromatin-fibrils of the 

 spindle may possess some chromatin, which collects upon their centers 

 to form the equatorial plate. If Flemming's last observation concern- 



