37 2 Digby. — -The Cytology of Primula kewensis 
Then the univalent limbs of each bivalent chromosome diverge widely, 
and together they fill up the greater part of the nuclear cavity (Fig. 63). 
This is followed by a shortening and thickening of the chromosomes 
(Fig. 64), the spindle makes its appearance and the chromosomes attach 
themselves to the fibres (Fig. 66 ). A transverse section of an equatorial 
plate shows the typical nine chromosomes (Fig. 65). 
There is nothing further to add about the heterotype division ; this is 
followed by the homotype resulting in the formation of tetrads (PI. XLIII, 
Fig. 67). It is only when the tetrad nuclei begin to separate from one another 
that some of the cells atrophy. A relatively small proportion of the pollen- 
grains appear to be normal (Fig. 68), but there are always depauperate 
ones amongst them (Fig. 69). Some of the anthers have a far higher 
percentage of healthy pollen-grains than have others. 
In order to ascertain that apogamy is not responsible for the unaccount- 
able doubling in the numbers of the chromosomes the embryo -sacs of both 
P. kewensis (sterile) and P. kewensis (fertile) have been examined. In 
both cases everthing is regular. In P. kewensis (sterile) 18 chromosomes 
(Fig. 70) are found in the nuclei of the nucellus, and 9 in the nuclei 
of the embryo-sac mother-cell (Fig. 71). 
P. kewensis (seedling form). 
The striking and all-prevailing difference between P. kewensis (fertile 
or seedling) and those Primulas to which it is related is the fact that it has 
doubled its chromosomes. By some means, probably at the act of fertiliza- 
tion of the single pin flower, by pollen of the thrum flower, borne on the 
sterile stock, the 4 (x) number of chromosomes has been obtained, and this 
4 (x) number is characteristic of all its subsequent generations. Moreover, 
P. kewensis with the 4 (x) number is fertile, whilst P. kewensis with the 
2 (x) number is sterile. * P . kewensis (fertile) has therefore 36 diploid 
(PI. XLI, Fig. 6) and 18 haploid chromosomes (PI. XLIII, Fig. 96). 
This double number is continued in the variety P . kewensis farinosa 
(PI. XLIV, Figs. 107 and 108). 
A most interesting recurrence of the doubling of the chromosomes has 
reappeared in the Primula crosses made at Kew in 1910 (29). The result 
of P. verticillata crossed with P . floribunda isabellina is a hybrid, which not 
only resembles P. kewensis farinosa in its external characters, but also with 
regard to its nuclei, for it possesses the number of chromosomes character- 
istic of the fertile P. kewensis , namely 36 (yx) (Fig. 108) chromosomes, that 
is to say it has duplicated the parental number. Thus the variety P. kew- 
ensis farinosa has arisen as the direct result of a cross, and indirectly 
as the result of a cross which has passed through an intervening sterile 
period. In both cases the number of chromosomes has been doubled. 
The phenomenon of the sudden duplication of a chromosome number 
