380 Digby . — The Cytology of Primula kewensis 
were abortive. In a hybrid cotton plant obtained by crossing Gossypium 
Barbadense with G. herbaceum Cannon (2) observed that in the material 
collected in November and December most of the male cells were normal, 
whilst in the material collected in the spring amitosis occurred. Jenctc( 26 ) 
has worked out the ratio of the sterile to the fertile pollen-grains in many 
hybrids. In the hybrid Primula austriaca , a cross between P. pannonica 
and P. acaulis , he found that between 50 and 60 per cent, of the pollen- 
grains were sterile, and in the hybrid P. venusta , a cross between P. car- 
niolica and P. Auricula , that 42 per cent, were sterile. 
It is generally accepted that the 2 x number of chromosomes in the 
hybrid is equivalent to the sum of the number of parental chromosomes, 
and in most cases this statement can be corroborated. One of the most 
convincing examples is to be found in the eggs of Ascaris. Herla ( 25 ) 
discovered that some Ascaris eggs possessed three chromosomes, of which 
two were larger than the third, and that the small chromosome was always 
isolated from the others. He concluded that it must have arisen from the 
univalens parent, but he was unable to verify the hybrid parentage. Zoja 
( 41 ), later, definitely proved that the eggs with three chromosomes were the 
result of a cross between Ascaris megalocephala bivalens and A. megalo- 
cephala univalens. He confirmed Herla’s supposition that the small 
chromosome was the paternal one, and hence concluded that ‘la cromatina 
paterna e la materna restano indipendenti nel nucleo delle cellule em- 
brionali 
Again, the investigation of Rosenberg (33 and 34 ) on the hybrid Drosera 
obovata furnishes another convincing example of the fact that the hybrid 
possesses the sum of the ^ numbers of the parental chromosomes. In the 
hybrid the 10 (x) chromosomes obtained from the parent D. rotnndifolia 
pair with 10 of the (x) number of the chromosomes of the parent D. longi - 
folia ; the remaining 10 chromosomes of D. longifolia remain unpaired. The 
ten bivalent chromosomes behave normally, but it seems to be a matter of 
chance to which pole the unpaired chromosomes repair, both in the heterotype 
and in the homotype divisions. Consequently the resulting tetrads possess 
a variable number of chromosomes. The pollen, as might be expected, is 
usually sterile. After several attempts Rosenberg has successfully crossed 
D. obovata with D. longifolia ( 34 ), and the offspring has about 37 chromo- 
somes. 
The recent work of Geerts ( 20 ) on the hybrid resulting from the cross 
between O. lata and O. gigas , and his interpretation of the union and 
behaviour of the parental chromosomes, are in direct accordance with 
the phenomena exhibited by the Drosera hybrid. On the other hand, 
Gates ( 16 ) has come to different conclusions as regards the behaviour of 
the chromosomes in the hybrid O. lata x O. gigas. He agrees with 
Geerts that, in the heterotype prophase of the hybrid, the chromosomes 
