326 Mottier — The Development of the Heterotypic 
however, no very good reason to believe that these steps represent other 
than normal stages through which all nuclei of the pollen mother-cells 
of Lilium pass, as well as in many other plants. At the stage of Fig. 28, 
the double spirem is slender, having shortened and thickened very little, for 
the period of greatest shortening and thickening begins with, or after, the 
cross segmentation. The processes following the condition of the spirem 
shown in Fig. 28 result in that illustrated by Figs. 29, 30, 31. From these 
figures it is clear that adjacent and parallel portions of loops approximate 
side by side and in many cases twist about each other. The degree of 
approximation and twisting of portions of the spirem always varies in the 
same nucleus. Also the regularity in the arrangement of the loops and 
other parts of the spirem varies greatly in different nuclei. Diagrammatic 
regularity is the great exception. The figures commonly seen are shown 
in Figs. 29, 30, 31. The most regular cases were not selected for illus- 
trations, and in the figures cited the writer has endeavoured to represent 
average conditions observed in the majority of nuclei. In Fig. 29, for 
example, the looping is much less regular ; three or more smaller nucleoli 
are present, and a larger part of the thread is knotted and entangled in the 
central mass. At this stage it is not possible to say that the cross seg- 
mentation is complete, but it is certain that some of the chromosomes are 
differentiated (Figs. 30, 31). In the light of these facts, it is scarcely 
possible to conclude that parallel parts of the loops, which are more or less 
twisted about each other, represent the halves of the spirem brought about 
by the longitudinal fission. In some cases it is to be seen that each parallel 
part of the loop is double, such a double nature being due to the longitudinal 
fission (Fig. 30). When the stage in the prophase represented by Figs. 30 
and 31 is reached, it is probable that cross segmentation is almost com- 
pleted. Some of the chromosomes, perhaps the majority that are loops, 
have the bend of the loop directed toward the nuclear periphery, but others 
are oriented with the free ends thus situated. All of the chromosomes are 
not formed as loops ; many appear as two rather straight or bent pieces 
free at both ends, which have come to lie side by side (Figs. 31, 32). 
Whether the approximation in these cases came about before complete 
cross segmentation cannot be stated. In the stage shown in Figs. 30 and 
31, the chromosomes are connected with each other and with the nuclear 
membrane by very delicate linin fibres. It is true, of course, that the 
chromatin is always in communication with the nuclear membrane and 
cytoplasm by means of such fibres. 
Following the stages of Figs. 30 and 31, the chromosomes separate 
from each other and tend to distribute themselves within the cavity of the 
nucleus. At the same time each contracts rapidly, becoming thereby 
shorter and thicker, and the fact that the two pieces of each chromosome 
are split longitudinally can no longer be definitely recognized, although 
