MITOSIS IN POLLEN MOTHER-CELLS. 2j 



THE SECOND OR HOMOTYPIC MITOSIS. 



In the pollen mother-cell of Lilium, the daughter nucleus does not 

 pass into the complete resting stage, although in some cases the 

 chromatin tends to become reticulated. In the homologous division 

 in the embryo-sac, the daughter nucleus, on the contrary, passes into 

 a structure which approaches closely that of the resting condition. In 

 Tradescantia the chromatin of the daughter nucleus reticulates more 

 than in Lilium while in certain dicotyledonous species, e. g., Lirio- 

 dendren and Magnolia (Andrews, '01), a complete resting condition is 

 reached. 



The spindle in Lilium and in all other plants investigated by the 

 author arises also as a multipolar complex of fibers. The develop- 

 ment of the multipolar structure and its transformation into the typical 

 bipolar spindle differ in no essential from that already described for 

 the first mitosis. 



In Lilium, it is very evident that the spirem does not segment 

 completely into chromosomes before the disappearance of the nuclear 

 membrane. The spirem does not split longitudinally in this division, 

 since that part of the process was accomplished in the preceding 

 mitosis, but during the transformation of the multipolar into the 

 bipolar spindle the chromatin skein segments into the chromosomes, 

 which are arranged in pairs in the nuclear plate. 



Within the complex of spindle fibers, the spirem, or pieces of it, 

 provided it has partly segmented, are somewhat crowded together. 

 The various turns are greatly entangled, kinked and knotted, so that 

 the segments cannot be accurately traced out. In only the most 

 favorable cases at this stage can a few segments or parts of the 

 spirem be followed definitely throughout their entire length (Fig. 1 1 , A). 

 The kinked and entangled condition of the skein or its segments is due 

 doubtless to the irregularity of the spirem, for were the turns all of a 

 uniform shape and size a less complicated arrangement would result. 

 The appearance of the chromatin during the development of the spindle 

 suggests that the chromosomes were brought to a more regular arrange- 

 ment in the nuclear plate by a pushing and pulling of the fibers. 



Judging from the form of certain chromosomes which stand out by 

 themselves, and which can be traced throughout their entire length 

 during the development of the spindle or in the nuclear plate, it seems 

 that the spirem, or a part of it at least, segments into pieces compris- 

 ing the two segments of a chromosome, i. e., the two granddaughter 

 chromosomes of the first division, and that these pieces may correspond 

 to long turns or loops of the spirem (Fig. 1 1, B, C). These loops are 



