648 Trow . — The Karyology of Saprolegnia. 
and mycelium, and the products of these divisions ultimately 
pass into the sporangia and gametangia. 
3. Neither nuclear divisions nor nuclear fusions take place 
in the sporangia. 
4. In the oogonia and antheridia each nucleus undergoes 
one reducing division by an indirect method, in virtue of 
which the whole chromosome becomes a half-chromosome, 
but no fusions of functional nuclei take place. 
5. The number of gameto-nuclei produced in the oogonium 
by the reducing division is about twenty times greater than 
that necessary to provide one nucleus for each oosphere. 
The number is reduced by the degeneration of the excess. 
6. Most of the gameto-nuclei in the antheridia and fertiliza- 
tion-tubes also undergo degeneration. 
7. Fertilization takes place invariably in 5 . dioica , and at 
least occasionally in N. mixta , Achlya americana , and another 
species of Achlya , while 5 . Thureti is normally apogamous. 
The whole-chromosome condition is restored to the nucleus 
either by the sexual process or by a process of growth. The 
two gameto-nuclei do not fuse to form the single zygote 
nucleus until a late stage in the maturation of the oospore. 
The Saprolegnieae as a group is not apogamous. 
8. At the period of germination of the oospore the nucleus 
of the zygote undergoes direct division to furnish one nucleus 
for each zoospore. 
9. The sporophyte-generation of the higher plants probably 
owes its origin to the fusion of gametes whose nuclei did not 
undergo a reducing division. The doubling of the chromo- 
somes acted as a stimulus to spore-formation, and involved 
a halving of the chromosomes in the return to the undoubled 
condition of the nuclei in the gametophyte-generation. 
University College, Cardiff, 
September , 1895 . 
