186 



THE CELL 



Fig. 79. Diagrammatic repre- 

 sentation of the segmentation of the 

 nucleus (after Flemming). Stage 

 in which the nuclear segments are 

 ai ranged in the equator of the 

 spindle. 



thicker, and are grouped around the spindle in the form of a com- 

 plete ring, the arrangement being that described by Flemming as 



the mother-star. The loop-like shape 

 of the segments is now most clearly 

 defined. They are invariably so ar- 

 ranged that the angle of the loop is 

 directed towards the axis of the 

 spindle, whilst its arms point towards 

 the surface of the cell. All of the 

 twenty-four loops lie pretty accu- 

 rately in the same plane, which, since 

 it bisects the spindle at right angles, 

 is called the equatorial plane; it is 

 identical with the plane of division 

 w r hich develops later. When seen 

 from either of the poles the chromatin 

 figure has " the shape of a star whose 

 rays are formed of the arms of the V-shaped loops, and whose 

 centre is traversed by the bundle of achromatin fibrils which 

 compose the nuclear spindle." This point of view is the most 

 convenient one for counting the nuclear segments, and for de- 

 termining their number to be twenty-four. 



Another most important process occurs during the second stage. 

 If the nuclear segment of a well-preserved preparation be ex- 

 amined with a high power of the microscope, it will be seen that 

 each mother segment is cleft longitudinally, and is thus split up 

 into two parallel daughter segments, which lie close together. 

 Now since no sign of this longitudinal division could be seen in 

 the original nuclear network, it follows that it must have occurred 

 after karyokinesis had commenced. Generally the longitudinal 

 cleft may be first seen w r hen the nuclear threads have arranged 

 themselves in the form of a coil (Fig. 75 B), but it is always 

 completed during the second stage (mother-star), when it is most 

 clearly defined. This was first observed by Flemming (VI. 12, 

 13), in Salamandra ; and his statements have been corroborated 

 by v. Beneden (VI. 4a), Heuser (VI. 39), Guignard (VI. 23), 

 Rabl (VI. 53), and many others, who made observations upon the 

 same and other objects. This longitudinal splitting appears to 

 occur invariably in indirect nuclear division, and is of the greatest 

 importance for the comprehension of the process, as will be 

 shown later on, when the subject is discussed theoretically. 



