Gates . — Somatic Mitoses in Oenothera. 
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very probably arise during meiosis, for one of the striking features of 
diakinesis in O. Lamarckiana and its mutants is that the chromosomes are 
but loosely paired (cf. Fig. 38), and there is, therefore, exceptional oppor- 
tunity for the occurrence of irregularities in the distribution of the meiotic 
chromosomes. I further showed that during meiosis in the pollen mother- 
cells such distributional irregularities actually do occur. 
From all the counts thus far made of the chromosomes in O. lata 
it appears that the number 1 5 occurs at least in most individuals, though the 
counts are perhaps not yet numerous enough to show that 15 is the number 
for all individuals, and that the O. lata characters are therefore constantly 
associated with the extra chromosome. The inconstancy of this mutant 
when self-pollinated (in a fertile race) or crossed may perhaps be accounted 
for by the presence of the extra chromosome, for it is probable that part of 
the megaspores produced will have 8 chromosomes, and part 7. 
Somatic Mitosis. 
Frophase. 
Though the somatic cells of Oenothera are relatively small, yet the 
nuclear structures are very clear, and in any meristematic tissue it is easy to 
get a complete series showing every stage of mitotic division within a small 
area. It is not to be expected, however, that the finest details of the 
chromatin in the earliest prophase stages can be depicted as minutely as in 
larger nuclei, so that such questions as the earliest stages of the trans- 
formation of the nuclear reticulum into the prophase chromosomes will not 
be critically considered here. The later prophase stages are, however, 
remarkably clear, and admit of no doubt in interpretation. The following 
description and figures refer only to the cells of the nucellus, though in other 
tissues the processes are essentially the same. 
When in the complete resting condition (PL LXXXVI, Fig. 1) the nucleus 
contains a delicate reticulum whose meshes are equally distributed through 
its whole cavity. Several nucleoli of varying sizes usually are present, and 
a small area more or less free from threads is usually found around the 
nucleoli, especially the larger ones, in fixed material. The reticulum in 
fixed preparations consists of the usual delicate interwoven threads, having 
a more or less beaded appearance owing to the presence at intervals of 
deeper staining and sometimes thicker portions of the threads, the same 
structures occurring also where threads cross. This moniliform appearance 
of the threads becomes more conspicuous at the beginning of the prepara- 
tions for prophase (Fig. 2). The threads also begin to lose their uniform 
character, some threads becoming visibly thicker and more conspicuous, 
while others retain the delicate character of the completely resting nucleus. 
Very soon this differentiation becomes more marked (Fig. 3), a few of the 
threads retaining their original delicate character, but most of them becoming 
