240 
Allen . — Nuclear Division in the 
de Vries has suggested, the object of the lateral apposition of paternal and 
maternal chromosomes is an exchange of pangens, and that as a result of 
the heterotypic and homoeotypic divisions each germ-cell contains only 
paternal or only maternal chromosomes. 
Very interesting phenomena have been observed by Rosenberg (’04 £) 
in the pollen mother-cells of Drosera. He finds that shortly after synapsis 
the short chromosomes lie side by side in pairs ; at first the two of a pair 
may be some distance apart, but soon they come into close contact with 
each other. The number of pairs is the reduced chromosome number. 
A fission of each chromosome occurs early, sometimes before the two of 
a pair are in contact ; this fission may be parallel or at right angles with 
the plane of contact of the chromosomes. The heterotypic division 
separates the chromosomes of a pair ; and the separation in the homoeo- 
typic division is in the plane of the fission just mentioned. Rosenberg 
(’03, ’04 a) has also studied a hybrid between Drosera rotundifolia and 
D. longifolia . The reduced number of chromosomes in the parents is 
respectively ten and twenty ; thirty is the number in the somatic cells 
of the hybrid. In the heterotypic division in the pollen mother-cells 
of the hybrid twenty chromosomes appear, ten of which are double, ten 
single. The ten double chromosomes divide in the usual way ; the single 
chromosomes either pass undivided to one or the other pole, or else are 
left on the 'spindle and take no part in the formation of the daughter 
nuclei. From the occurrences in the pollen mother-cells of the pure 
species, it must be inferred that each double chromosome in the hybrid 
results from the conjugation of a maternal and a paternal element ; while 
the single chromosomes are those derived from the longifolia parent which 
have failed to find mates among the smaller number from the rotundifolia 
parent. This inference is confirmed by the fact that the chromosomes 
of D. rotundifolia are much larger than those of D. longifolia ; and that 
each double chromosome in the hybrid consists of a larger and smaller 
daughter segment. 
Berghs (’04) has studied the heterotypic and homoeotypic divisions 
in the microsporogenesis of Allium fistulosum and L ilium speciosum ; and 
his latest results, as reported by Gregoire (’04), agree very closely, as to the 
fusion in pairs of the strands of the spirem during synapsis, the double 
longitudinal splitting, and the subsequent history of the chromosomes, with 
my observations. Both of these authors seem inclined to deny the 
formation of a single continuous spirem, although Gregoire (p. 308) says 
that after the completion of the fusion the spirem appears to be unseg- 
mented. They consider the first longitudinal splitting as merely the 
separation of threads which have been for a time in contact but have 
retained their autonomy. 
The apparent difficulty of reconciling the notion of a double longi- 
