356 Olive . — Sexual Cell Fusions and Vegetative 
differentiation. ( 3 ) The gamete which bears the degenerating tip-cell often 
appears to be placed somewhat above the other, thus suggesting that the 
earlier hyphae fuse, not among themselves, but with other hyphae which 
push up later from below. 
This explanation appears simpler and more reasonable to the writer 
than the complicated theory proposed by Blackman. The sterile cell, 
according to the views advanced in the present paper, is not an abortive, 
functionless trichogyne, but merely a c buffer cell ’ — a degenerate gameto- 
phytic cell, morphologically similar to the functional gametes. The simpler 
theory of course leaves the so-called spermatia as yet unexplained. 
Conjugation is accomplished, as Christman maintains, through the 
absorption of a portion of the walls of the two gametes which are in contact. 
The process may begin, however, through a very small conjugation pore, 
so that as the one protoplast moves through the narrow opening to fuse 
with the adjoining gamete, the nucleus may thus sometimes be drawn out 
and constricted ; in this condition presenting the appearance which Black- 
man has termed ‘ nuclear migration ’. Such an instance is regarded, 
however, merely as a case of conjugation of two cells in which the connect- 
ing pore is as yet small. Instances were observed in which a Blackman 
type of conjugation, as it may be termed, through a narrow pore, occurred 
side by side with a Christman type of fusion, through a broad pore. The 
essential feature of the conjugation process in the caeoma forms is therefore 
regarded as the equal participation of two morphologically equivalent cells 
to form a binucleated double cell — the so-called ‘ fusion cell ’. 
2. In the present investigation, the vegetative nuclear divisions have 
been studied in the spermogonia and in other gametophytic hyphae, as well 
as in the binucleated sporophytic hyphae. The process is essentially the 
same in all these types of cells, being a mitotic phenomenon, and not par- 
taking of the nature of amitosis, as Blackman claims for the nuclei of 
Phragmidium. Each nucleus, during the conjugate divisions, acts appar- 
ently in entire independence of its associated nucleus. During the earlier 
stages of the association of the two nuclei in the one cell, just following the 
sexual fusion, the mitotic figures may sometimes be variously oriented 
in the cell, bearing no obvious relation to each other. Later, however, the 
two conjugate spindles generally arrange themselves more or less regularly 
parallel to each other in the long axis of the cell. 
Each nucleus divides by the aid of a centrosome, which is located 
on the nuclear membrane, and which in some forms persists in the resting 
stages as a distinct point of polarization of the nuclear contents. During 
the prophases, the centre divides and the two daughter-centres move apart 
on the nuclear membrane. Between the diverging centres a filamentous 
structure appears — the central spindle of Hermann. Soon the nuclear 
membrane breaks down and the central spindle comes to lie as a dense, 
