We must note, as bearing on this point, that while the process of
fertilization, in all cases where it has been thoroughly investigated, is
the formation of a cell with the double number of chromosomes, the
combination of these chromosomes in the single nucleus may be either
immediate or a more gradual process, as is conspicuous in the embryos
of Cyclops; and Blackman's (8) and Christman's (15) interesting dis-
coveries in the aecidium of the rust show that the final combination of
the nuclei may be delayed through an indefinite number of cell genera-
tions. The ultimate fusion in this case seems to be associated with the
process of chromosome reduction and the development of spores.
Blackman's discovery of his so-called vegetative fertilization in the
aecidium of the rusts, bearing as it does on the whole question of the
homologies and relationships of the higher fungi, as well as on the
general question of fertilization and alternation of generations, is cer-
tainly of the highest importance. With his account and that of Christ-
man we may believe that the historic question as to the sexuality of the
aecidium is finally settled. Blackman finds that the basal cell of each
row of aecidiospores in Phragmidium violaceum is fertilized by the
migration through its wall of a nucleus from a neighboring cell. The
pronuclei do not fuse, but divide by conjugate division, and thus the
cycle of binucleated cells which terminates with the nuclear fusion in the
teleutospore is begun. There is no wide communicating pore between
the gametes, as found by Blackman. The nucleus passes through the



NUCLEAR FUSION IN THE ASCUS. 63

wall without leaving a trace to indicate its path. The fertilization is
accomplished by the nucleus alone.
