952 Davis.—A Comparison of the Reduction Divisions of 
smaller numbers of chromosomes, and a similar case was also observed by 
the same author (Gates, ’ 08 ) for rubrinervis . Both Geerts and Gates report 
evidence of nuclear degeneration in stages previous to the heterotypic 
mitosis, indicating that the causes of sterility are more deep-seated than 
irregularities of chromosome distribution, although the latter phenomenon 
is perhaps intimately associated as an effect of physiological disturbances 
previous to pollen formation. 
The writer can only report for Lamar ckiana the facts of certain 
irregularities in the distribution of the chromosomes. Thus, during the 
homotypic mitosis it is not uncommon to find that some of the chromosomes 
in a group of seven have failed to reach the poles of the spindle, and as 
a result form smaller supernumerary nuclei in the pollen mother-cell. Such 
a case is shown in PI. LXXII, Fig. 44, where the chromosomes of three 
groups, a total of twenty-one, are distributed among five nuclei. Tetrads 
may even be formed in which large and small nuclei become associated in 
the same cell and pass into a resting condition, but it is not known whether 
such a cell can mature into a fertile pollen-grain. An irregularity in the 
heterotypic mitosis has also been described (Fig. 30) in which six or eight 
chromosomes respectively had been distributed to opposite poles, but here 
also we do not know whether or not fertile pollen-grains may be formed with 
chromosomes in a greater or less number than the normal. 
An extensive field for cytological investigation is suggested by the 
recent paper of de Vries (’ll) on double reciprocal crosses between Oeno¬ 
thera biennis and O. muricata. In explanation of his remarkable results 
de Vries postulates the development of two forms of germ cells, both male 
and female, carrying two different sets of hereditary tendencies. One of 
these two forms of gametes is eliminated from the male organ by the 
sterility of half of the pollen-grains, and the other form is eliminated from 
the female organ by the abortion of half of the ovules. As a result two 
groups of characters are strictly sex-limited in their inheritance, one being 
carried through the male and the other through the female gamete. It is 
too early to discuss this hypothesis from the cytological side, for the 
problems concerned will demand prolonged investigation. The writer’s 
belief, based on preparations of pollen in which the entire contents of 
anthers were examined, is that the proportion of sterile pollen varies 
widely. Thus in certain races of Oenothera biennis from 20 to 30 per cent, 
of the pollen is sterile, while in other races the proportion of sterile pollen 
appeared to be fully 75 per cent. 
It seems clear, however, that the total amount of sterility in the 
Oenotheras is much too great to be explained entirely by peculiarities 
of the chromosomes, or by such irregularities in their distribution as are 
known to occur during the reduction divisions. Thus, it is difficult to 
understand how the ovules of Oenothera lata can be fertile and the pollen 
