Pollen Mother-cells of Lilium canadense . 207 
disappearance of the latter. In each of these respects her description is 
inapplicable to Lilium canadense . The transition to the synapsis-like 
figure occurs here before segmentation, or when that process has barely 
begun. The nuclear membrane remains distinct, the tangled mass is 
always central instead of peripheral, and the nucleoles retain in general 
their rounded form. Occasionally a nucleole is seen that is elongated or 
somewhat irregular in outline, but never at all approaching the curious 
4 sickle shape ’ of synapsis. The nucleoles sometimes appear vacuolated, 
sometimes homogeneous. I have seen no evidence in this or in any other 
stage of any genetic connexion between the material of the nucleoles and 
that of the chromosomes. 
Ernst (’02) also describes and figures a stage of aggregation following 
segmentation in the macrospore mother-cells of Paris and Trillium , but 
he confuses it with the much earlier synapsis, properly so-called, which 
stage he seems not to have observed. 
Schaffner (’ 97 ) finds, in the macrospore mother-cell nucleus of Lilium 
philadelphicum , that after longitudinal fission the chromatin thread becomes 
arranged in twelve loops, the ‘ head ’ of each loop being near the nuclear 
membrane. Segmentation occurs by the breaking apart of these loops ; 
that is, the transverse breaking occurs in the central part of the nucleus, 
so that the peripheral portion of each loop becomes the central portion of 
one of the newly-formed chromosomes. The same arrangement of the 
spirem and the same method of segmentation are described as characteristic 
for the heterotypic divisions of both animals and plants in a recent paper 
by Farmer and Moore (’ 08 ). My figures agree both with those of Schaffner 
and with those of Farmer and Moore as to the formation of the loops; my 
series of preparations, however, makes it certain that one of the loops does 
not represent that part of the spirem which is destined to form a chromo- 
some, but that, on the contrary, the peripheral portion of the loop marks 
the region in which the separation between two adjacent chromosomes 
is to occur. While the general arrangement of the segmenting spirem is 
as I have just described, it is subject to occasional variation. For instance, 
in Fig. 29 one newly-formed chromosome (aa) lies comparatively free from 
the central mass and can be traced throughout its length. A single 
chromosome from a nucleus at about the same stage is shown in Fig. 33 ; 
but on account of the closeness of the central mass and the usual arrange- 
ment of the chromosomes with reference thereto, it is only in exceptional 
cases that a chromosome can be so followed for its entire length. Between 
the time of longitudinal fission and that of segmentation there is little 
if any increase in the thickness of the thread ; but as soon as the free 
ends begin to appear, the process of thickening seems to go on more 
rapidly, as well as somewhat unevenly, so that a decided difference in 
thickness becomes apparent, even between different parts of the same 
