and of other related Primula Hybrids. 383 
of P. kewensis , the duplication is accompanied by the acquirement of the 
character of fertility. 
Further, both in the Oenotheras and in the Primulas, an example of 
a decrease in the chromosomes is found, whereby the duplicated number is 
reduced to the typical number. In the Oenotheras this reduction has been 
shown to be achieved by the actual disintegration of the supernumary 
chromosomes. It may be that in the hybrid produced by crossing P. flori- 
bunda isabellina with P. kewensis (seedling form) a similar elimination 
of chromosomes has taken place in the first segmentation division after 
fertilization. 
Summary. 
1. The parents, P. Jloribunda and P. verticillata of the sterile hybrid 
P. kewensis have both 18 ( 2 x) and 9 (x) chromosomes. 
This number is repeated in the sterile hybrid P. keivensis. 
P. kewensis (seedling form), which originated from P. kewensis (sterile) 
by the fertilization of a pin flower with pollen of a thrum flower (both on 
the sterile stock), has doubled the typical number of chromosomes and 
possesses 36 (2 x) and 1 8 (x). 
This number is repeated in the variety of the seedling form, P. kewensis 
farinosa , and reappears in the P. kewensis farinosa , which resulted from 
crossing P. verticillata with P . Jloribunda isabellina. 
P . jloribunda isabellina crossed with P. kewensis (seedling form) results 
in a hybrid which has reduced its chromosomes to the typical number of 
18 (2 x) and 9 (x). 
2 . The prophases of the premeiotic divisions show a stringing together 
of homogeneous beads of chromatin, which gradually condense to form the 
chromosomes. Longitudinal fission in the substance of the chromosomes 
is seldom seen until the chromosomes are arranged on the spindle. 
3. The long rest between the last premeiotic and the first meiotic division 
makes it impossible to trace the homology of the parallel threads of the 
heterotype prophases. 
4. In P . Jloribunda and in P. kewensis (seedling), there is no second 
contraction. Hence the sequence of events leading to the formation of the 
bivalent chromosomes can be traced without intermittence, namely, the 
looping of the continuous univalent spireme ; the approximation and 
association of the univalent sides of the loops ; and the final splitting apart 
of the approximated sides to form the univalent chromosomes of the bivalent 
combination. 
5. This series illustrates clearly the telosynaptic arrangement of the 
univalent chromosomes in the continuous univalent spireme of the hollow 
spireme stage of the heterotype division, and the secondary parasynaptic 
union of homologous univalent segments to form the future bivalent 
heterotype chromosomes. 
