202 Sargant . — Formation of the Sexual Nuclei 
had been divided into chromosomes. Thus each chromosome 
has been divided into two segments from the very outset of 
its independent existence, and these segments were formed 
by a process of longitudinal fission. 
It is a remarkable fact that in such an immature chromo- 
some as that drawn in Fig. 9 a , each segment exhibits a fission 
of chromatin-granules exactly resembling that which pre- 
ceded the complete fission of the whole chromosome. We 
are naturally led to expect that this also will be completed 
at a later stage of development, and that such a chromosome 
as we have described has an essentially fourfold structure. 
This view is the more tempting when we recall the formation 
of tetrad-groups, which is so characteristic of the animal sper- 
matocyte. Dr. Brauer has shown that in the case of Ascaris 
each tetrad-group is the result of a double longitudinal fission 1 . 
The chromosomes of the pollen-mother-cell nucleus of 
Lilium Martagon , however, undergo a change, on the final 
disappearance of the nuclear membrane, which effectually 
conceals all traces of the incomplete fission just described. 
Up to the time of that disappearance the segments of each 
chromosome show the differentiated structure drawn in 
Fig. 9 a. But as the membrane vanishes, the colouring of 
the chromosome-segments becomes uniform. Each is appa- 
rently homogeneous. There is no contrast between cyano- 
philous granules and erythrophilous ribbon, but the whole 
chromosome stains uniformly like chromatin (Fig. 11). A 
similar change has been observed at the corresponding period 
in the history of the embryo-sac nucleus (I, p. 461). 
The true structure of the mature chromosome is very much 
obscured by this change in colouring power. However tightly 
its segments were twisted on each other, they could still be 
distinguished without difficulty so long as each consisted of 
a ribbon outlined with a dark border of dots. Now, however, 
the segments are indistinguishable where they cross each 
other. The loops between them are sometimes fairly large, 
1 Brauer, Zur Kenntniss der Spermatogenese von Ascaris megalocephala, Arch, 
f. mikr. Anat., V. 42, 1893. 
