150 



INTRODUCTION TO CYTOLOGY 



homogeneous, but for reasons which will be given later it is evident that 

 its substance has a definite longitudinal orientation of some kind. The 

 fine "fibrils" or ''lamellae" seen running from pole to pole in fixed 

 preparations are a visible modification of this oriented structure. In 

 addition to these fine fibrils, there are coarser ones which begin to develop 

 at the attachment regions of the chromosomes and extend poleward 

 along the spindle; these have often been called "tractile fibers." 



In the anaphase the halves of each split chromosome begin to move 

 apart at the spindle-attachment region. The tractile fibers are very 

 prominent (in fixed preparations) at this time, but when the chromosomes 

 reach the poles they soon disappear. The spindle substance extending 

 between the two chromosome groups still shows its characteristic fine 



.^•i7- 







.-> 



;-;^^Q^;- 



:• ••^*' )Vy 







1 2 3 4 



Fig. 81. — Development of achromatic figure in root tip of Hyacinthus orientalis. 

 1, nucleus after reaching its maximum size in prophase. 2, nuclear membrane shrunken, 

 leaving polar caps. S, membrane partly gone; caps about to become spindle cones. 4< 

 spindle established; metaphase. Note that chondriosomes do not invade the spindle. 

 (After Robyns, 1924.) 



striations. As the two groups develop into telophase nuclei, this mass 

 of substance widens in the equatorial region to form a barrel-shaped 

 phragmoplast, which usually continues to extend laterally until it comes 

 in contact with the wall at one or more sides of the cell. It seems prob- 

 able that a portion of the spindle substance or some constituent of it 

 reenters the growing daughter nuclei while the remainder is functioning 

 as the phragmoplast. 



While the above changes are taking place, the partition which is to 

 divide the original cell into two begins to develop. There is first formed 

 a delicate membrane known as the cell-plate through the equator of the 

 phragmoplast. The time at which it appears with reference to the 

 nuclear changes varies considerably in different cases. Commonly it 

 appears first in the middle region and extends laterally as the phragmo- 

 plast widens. The phragmoplast, or spindle substance, soon disappears 



