MITOSIS IN POLLEN MOTHER-CELLS, 27 
THE SECOND OR HOMOTYPIC MITOSIS. 
In the pollen mother-cell of Ze/éum, 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 Z2?ém while in certain dicotyledonous species, e. g., Lérdo- 
dendren and Magnolia (Andrews, ’o1), a complete resting condition is 
reached. 
The spindle in Zzdéam 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 Lelium, 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. 11, 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, 7z. e., the two granddaughter 
chromosomes of the first division, and that these pieces may correspond 
to long turns or loops of the spirem (Fig. 11, B,C), These loops are 
