Nuclear Divisions in the Rusts. 
355 
which follows the sexual cell fusions, or whether they are formed during the 
gametophytic stage, preceding the fusions, I am not prepared to say. The 
studies so far made on this point are only partly convincing ; but, as men- 
tioned above, the few facts at my command incline me to the belief that 
the multinucleated cells are sporophytic structures and that they result from 
the stimulated growth that follows the sexual cell fusions. Leaving aside 
for the present, however, the question of their origin, it is quite obvious that 
their seemingly normal and regular occurrence during the development 
of the young aecidia argues strongly against the acceptance of either 
Blackman’s or Christman’s ideas as to the mode of origin of the binucleated 
condition in the aecidium-cup Rust. At any rate, it is evident that further 
investigations of the minute structure and development of further types of 
the aecidium is needed before we can hope to bring this more complicated 
type into relation with the simpler caeomas. 
Summary of Results and Conclusions. 
i. The seemingly conflicting results obtained by Blackman and 
Christman, in their investigations of the sexual phenomena in the caeoma 
type of Rusts, are to some extent brought into harmony by certain new and 
supplementary facts recorded in the present paper. The two fusing 
gametes, as well as their nuclei, are regarded, however, as approximately 
equal, therein affording in greater part a confirmation of Christman’s con- 
clusions. While agreeing also with some of Blackman’s observations of 
facts, certain of his conclusions with regard to these observations are not 
supported ; e. g. that one of the fusing cells is generally smaller than the 
other, and that this smaller cell, which he thought contained a smaller, 
denser nucleus, is to be regarded as a £ vegetative cell as distinct from the 
larger ‘ fertile cell ’. Blackman’s further theory that his so-called larger cell 
constitutes a ‘ female cell ’, and that its sterile tip is phylogenetically a 
‘ trichogyne ’ also lacks support in the facts brought out in this investi- 
gation. 
While the conjugation is therefore regarded as taking place between 
two gametes which are essentially similar in size, several observations in 
connexion with the sexual fusions, apparently noted only in part by Black- 
man and Christman, point to the conclusion that the two gametes differ 
somewhat in time of development. The observations on which this con- 
clusion is based are as follows : (i) In the caeoma forms the first hyphae 
to push up under the epidermis mass themselves often more or less regularly 
upright and parallel and then proceed to cut off sterile cells at their tips. 
The sterile tips push up against the epidermal cells and soon degenerate. 
A more or less prolonged period of vegetation appears thus to intervene 
before the conjugations begin. (2) Generally only one of the two con- 
jugating gametes bears such a sterile tip, while the other shows no such 
