Chromosomes in Pollen Mother-cells. 
33i 
critical studies of the resting nucleus. The interpretation of the structure 
and origin of the chromosomes was based upon the form and arrangement 
of these structures as they appeared just previous to, or upon, the spindle, 
and their subsequent behaviour during the second or homotype mitosis. 
The prevailing view of reduction at the time, namely, that this was 
accomplished during the second division, centred attention largely upon 
the second and upon the latter part of the first mitosis, with the very 
important result that the second was not the reducing division. The 
discovery of the Mendelian principle of the segregation of characters in 
hybrids stimulated cytologists to a renewed study of the first mitosis with 
present results. 
The more recent study of the presynaptic phase has suggested new 
problems and new interpretations, not the least important of which is the 
individuality of the parental chromosomes in somatic cells, and their 
recognition in the resting nucleus. The writer will now endeavour to 
show what bearing the facts observed in the plants in question have upon 
some of the explanations that have been proposed. 
The nucleus is almost universally recognized as being in the resting 
stage when the chromatin is in a finely divided state, consisting of small 
granules, held in a linin network of delicate threads, in which none of 
the well-recognized phenomena of mitosis can be detected. Of course, it 
is not possible, and it may not be necessary, to define the term resting 
nucleus with strict accuracy. In different plants, and probably under 
different conditions in the same organ, the degree to which the breaking 
up into finer granules, or the reticulation of the chromatin has taken place, 
varies considerably. Sometimes it is certain that just prior to synapsis the 
reticulation may result in a very finely divided state, in which the granules 
are perhaps of the same size as, or even smaller than, one-half of the 
paired chromomeres which appear in the spirem as it merges from synapsis 
(Figs. 19, 20}. In other cases the granules are larger, and may be spoken 
of rather as lumps or masses than as granules (Figs. 22, 34). This latter 
condition, especially when the masses or lumps approach in number that of 
the somatic chromosomes, has been interpreted as indicating the nature in 
which the individual chromosomes are recognized in the resting nucleus, 
each lump being a ‘ prochromosome 5 or centre of organization of a chromo- 
some. Thus Overton (’ 05 ) claims that in the pollen mother-cells of 
Thalictrum purpurescens , Calycanthus floridus , Helleborus foetidus , and 
Campanula grandis , the number of these masses, or prochromosomes, 
equals the number of somatic chromosomes. Miyake (’ 05 ) suggests a 
similar interpretation in regard to certain monocotyledonous species, but 
he does not commit himself to this proposition, stating that it is not easy 
to determine the number of chromatin lumps, and that in Lilium the 
number of lumps exceeds that of the somatic chromosomes. My own 
