426 F. R. ROWLEY ON SOME POINTS IN THE STRUCTURE 



of the nucleoli which are still clearly visible (Fig. 6, c.s.). So 

 much alike, indeed, are these two cell constituents, that if the 

 origin of the spindle had not been observed it might have been 

 looked upon as a nucleolus which had migrated from the interior 

 of the nucleus. 



The changes in the interior of the nucleus are accompanied by 

 an alteration in its outward form, which becomes more or less 

 ellipsoidal instead of kidney-shaped. At the same time the 

 rearrangement of the chron)atin elements has advanced a step 

 further. The moniliform rows of granules have now commenced 

 to fuse together, forming twisted threads. A collection of granules 

 as yet practically unaffected remains in the centre of the nucleus, 

 but the nucleoli are disappearing. According to Lauterborn, they 

 probably contribute in some way to the formation of the changing 

 nuclear network. Lying obliquely midway between nucleus and 

 centrosome is the developing spindle, somewhat boat-shaped when 

 seen in optical section. 



As we shall see more clearly a little later, the entire proto- 

 plasmic contents of the Diatom cell commence to move towards 

 the broadest end of the frustule at the beginning of division, the 

 whole process strongly suggesting that the centrosome is exerting 

 an attractive influence upon the various cell contents, and is the 

 agent which determines the direction of movement. 



It is at this point that we notice the appearance in the 

 cytoplasm of the little rod-like bodies (" Doppelstabchen ") 

 previously mentioned. We now arrive at the comj)letion of what 

 is known as the spireme stage. The chromatic network of the 

 nucleus consists of many twisted threads, appearing in optical 

 section as scattered dots. The central spindle which we left as 

 a circular plate-like body is now rapidly developing. Its two 

 surfaces begin to recede from one another and the intervening 

 space exhibits a very delicate striation. At the upper extremity 

 of the spindle, on each side, two dark masses stand out clearly 

 which are destined to form the new centrosomes, one for each of 

 tne daughter cells. The original centrosome, after decreasing in 

 size, suddenly vanishes, and it has not yet been satisfactorily 

 ascertained whether it is dissolved, so to speak, in the cytoplasm 



