382 Digby . — The Cytology of Primula kewensis 
of this hybrid, which he calls .S. chinensis. Tischler (36) found in the 
hybrid Cytisus Adami , a cross between C. Laburnum and C. purpureus , 
most irregular divisions resulting in an unequal distribution of chromosomes 
to the poles. Again, in the hybrid Bryonia (37), a cross between B. alba 
and B. dioica , he has shown that the presynaptic and synaptic stages are 
normal, but that the chromosomes go on to the spindle irregularly, while 
some remain behind in the cytoplasm. The hybrid is always sterile. In 
the hybrid Ribes intermedium , a cross between R . sanguineum and R. nigrum 
(38), the meiotic phase is normal and 10 to 15 per cent, of the pollen-grains 
are well developed. 
The fact that a hybrid may be either sterile or fertile does not therefore 
appear to depend on the pairing of an equal number of parental homologous 
chromosomes, nor can the irregularity of the spindle figures be called 
a determinant of hybridity. It is true that irregular figures and the peculiar 
amitosis must entail an unequal partition of chromatin amongst the tetrads, 
and hence may render them functionless, but these abnormalities are not con- 
fined to hybrids, but may also occur in pure breeds. Juel (27) has emphasized 
this fact in the nuclei of Hemerocallis fulva . 
Tischler and others believe that the arrangement of the chromosomes 
cannot be taken into account with regard to the cause of sterility, but that 
the sterile condition is more probably due to the poverty of the cytoplasm 
and to the alteration in the constitution of the idioplasm. 
Most cytologists have arrived at the conclusion that, in the blending 
of the male and female germ cells, physiological and chemical changes 
are induced, which fundamentally disturb the metabolism of the cells of 
the resultant offspring. The abnormal nuclear phases, the anomalous 
behaviour of the chromosomes, and the feeble condition of the cytoplasm 
afford visible expressions of the influence of unrevealed agencies which 
effect these intrinsic changes. 
In conclusion, it is of interest to emphasize the points of similarity 
between this series of Primulas and the varieties of Oenothera . Briefly the 
character of the heterotype prophases, the presence in some forms and the 
absence in others of the second contraction, the longitudinal fission in the 
chromosomes throughout the interkinesis of the two meiotic divisions, the 
cytoplasmic connexions between the pollen mother-cells, are all features 
which the Primulas possess in common with the Oenotheras. 
Moreover, both in the Oenotheras and in the Primulas a form has 
arisen from different sources, which has suddenly doubled the number of 
the chromosomes characteristic of the type. From the Oenothera evidence it 
seems probable that this duplication has been brought about by longitudinal 
fission, creating what is believed to be a double set of homologous chromo- 
somes, for the duplication of the chromosomes has not been accompanied 
by the acquirement of any new characters. On the other hand, in the case 
