AND LIFE-HISTORY OF DIATOMS. 427 



of the cell, or whether it is merged in the substance of the two 

 growing centrosomes. 



The next phase in the process of division is marked by the 

 entry of the central spindle into the substance of the nucleus, 

 where it lies at first somewhat obliquely (Fig. 7, c.s.). At this 

 stage, also, the chromatic substance of the nucleus has split up 

 into separate pieces — the chromosomes (chr.) — most of which have 

 become loop-like. Not only so, but each chromosome about this 

 time splits lengthwise into two similar halves, and the first step 

 is thus taken towards the equal distribution of the chromatin 

 between the daughter cells which it is the object of cell division 

 to accomplish. 



A little later the central spindle takes up a position in the 

 middle of the nucleus, and at right angles to a line drawn 

 through the Diatom from end to end. Its curved extremities 

 (the poles of the spindle) are connected by numerous extremely 

 delicate threads (the interzonal fibres), which pass uninterruptedly 

 from pole to pole. The chromosomes, though not at first arranged 

 in any particular way, show an increasing tendency to collect 

 around the equator of the spindle (Fig. 8). 



Lauterborn states that it is impossible to follow individual 

 chromosomes in their migration towards the equator of the 

 spindle, i7itra vitam, owing to the indistinctness of their contours, 

 so that stained preparations are necessary for the study of the 

 process ; but so soon as they have arrived at their destination, 

 and are grouped together in a ring-like mass, this equatorial 

 ring, as well as the central spindle itself, stand out with extra- 

 ordinary distinctness. The mitotic figure when viewed endways 

 at this stage shows a clear, approximately circular central space 

 occupied by the interzonal fibres, around which the chromosomes 

 are grouped in a radiating fashion. 



Perhaps the most striking feature in connection with this 

 " Stadium des Muttersterns " is the way in which the chromosomes 

 appear for a time to almost entirely lose theii* individuality, 

 visually speaking, since only one or two can be seen faintly 

 outlined in the uniform greyish mass (Fig. 9). The nucleus is 

 said to remain for nearly an hour in this condition, wliich is 



