Spindle in Vegetable Cells of Higher Plants 457 



From the foregoing it will be seen that the early development 

 of the spindle does not quite agree with the observations of Hof 

 ('98) on Ephedra in which it is claimed that no trace of the spindle 

 appears until after the segmentation of the chromosomes. The 

 segmentation of the chromatin spirem into the individual chromo- 

 somes represents a definite phase in the development of the karyo- 

 kinetic process, yet the time when it occurs as measured by a cer- 

 tain stage in the formation of the spindle may not seem to be of 

 much importance since it has been shown that the time of seg- 

 mentation varies much under different conditions in embryo-sac 

 and in pollen-mother-cells and we find this to be true also in vege- 

 tative cells. 



(M 



embry 



before the nuclear membrane disappears and before a definite 

 spindle is present ; but in the two succeeding divisions in the 

 embryo-sac and the second one in the pollen-mother-cell, which 

 take place in a rather rapid succession, the nuclear membrane dis- 

 appears and a definite spindle may be developed before the seg- 

 mentation into chromosomes. 



The first nuclear division in the pollen- and embryo-sac- 

 mother-cells is preceded by a long period of growth in which the 

 nucleus is in a resting stage characterized by the fine nuclear re- 

 ticulum, whereas the second and succeeding divisions in the em- 

 bryo-sac of Lilium follow a much shorter period of rest. In the 

 pollen-mother-cell, as is well known, the nucleus does not enter 

 the complete state of rest at all before the second division. 



Hof has observed that in vegetative cells one nuclear division 

 may follow another in rapid succession without the intervention 

 of the period of complete rest, and we are convinced that the same 

 often happens in Allium. In the light of this fact, therefore, it is 

 possible that such conditions as revealed in Figs. 5, 9 and 10 

 represent nuclei which are dividing without having passed through 

 the stage of complete rest. Here as in the embryo-sac the spindle 

 has probably advanced farther before the segmentation of chromo- 

 somes. The possibility is also not excluded that in Fig. 14 we 

 have the chromosomes uniting to form a hollow spirem to be a 

 corresponding stage in a rapidly succeeding division. 



