12 



INTRODUCTION. 



contains a row of larger granules or disks (the Chromatinscheiben of 

 the German literature) of a tolerably uniform size, becomes a hollow 

 spirem whose irregular turns traverse the nuclear cavity (Fig. 5, C). 

 The chromatin disks have usually a jagged or erosed outline, which 

 shows that each disk is composed of smaller granules. The chromatin 

 disks, first carefully described by Strasburger ('82), vary much among 

 themselves in size, and do not always have the same orientation in the 

 linin thread. This fact, together with the twisting of the thread upon 

 its axis, which is a mechanical necessity, gives the impression of a 

 spirem composed of very irregular granules. This is especially notice- 



FIG. 5. Pollen mother-cell and early prophase of first or heterotypic mitosis. A, F, Podophylliim 



peltatum. B-E, Helleborus foetidus . 



A, typical pollen mother-cell, with nucleus in resting stage, and while the cells are in tissue connection. 



B, linin net with numerous small chromatin granules. 



C, spirem in which chromatin disks are of uniform size. 



D, pieces of chromatin spirem more highly magnified ; a, before longitudinal splitting; b, after longi- 



tudinal splitting. 



E, the spirem has split longitudinally; daughter segments show a tendency to separate. 



F, the chromatin spirem has segmented transversely into chromosomes; daughter segments twisted 



about each other. ( All figures represent sections.) 



able immediately after the longitudinal splitting of the chromatin 

 granules. At this stage the most careful staining is necessary to bring 

 out the chromatin disks clearly, since the linin retains the stain with 

 greater avidity, thereby concealing the former. If the nuclear thread 

 be too densely stained, it will appear more or less homogeneous, in 

 which case the chromatin disks manifest themselves as a succession 

 of enlargements whose granular character is concealed. The chro- 

 matin thread consists, therefore, not of a succession of chromatin disks 



