475 
Cytology of the Ascomycetes. 
out from the centrosome. Our present studies, especially on Peziza 
vesiculosa , have strengthened this point of view. It has been suggested 
that the centrosome is 'the seat of fermentive activities. On this 
hypothesis the centrosome, as it pushes outwards through the cytoplasm 
at the end of the third division, might be regarded as constantly generating 
a ferment. This ferment would flow back in its wake and would delimit 
the spore by producing a chemical change in the area through which it was 
distributed. It would ordinarily tend, as in the polar aster, to flow out 
equally all round the centrosome, but in this case, owing to the movement 
of its source, it would flow especially backwards. Its effect would be 
limited in certain directions by the occurrence of vacuoles and by the 
presence of the ascus wall. Whether the changes which thus take place 
are due to enzyme activity or to some other agent, we conclude that while 
the spore is to some extent bounded by neighbouring vacuoles, the main 
factor in its delimitation is an alteration of the cytoplasm, originating at 
the centrosome, and essentially similar in character to that which produces 
the aster. 
Summary. 
i. In O tide a aurantia traces of a probably functionless ascogonium 
are present ; no such structure was found in Peziza vesiculosa. 
<Z. In both species the first and second divisions in the ascus constitute 
a meiotic phase ; this was investigated in some detail in Otidea aurantia , 
and was found to correspond closely to the description given by Farmer 
and Moore, the first division being diaschistic. 
3. Fusion in the ascus occurs at about the time of the first meiotic 
contraction. 
4. A second reduction takes place in the third division in the ascus. 
In Otidea aurantia the chromosomes pair in the prophases of this division ; 
in Peziza vesiculosa they unite at an earlier stage. 
5. We regard the presence of both meiosis and brachymeiosis in these 
forms, as well as in those previously described, as additional evidence 
of the occurrence of two fusions in the life-history of Ascomycetes. 
<5. The number of chromosomes in the first division in the ascus is four 
in Otidea aurantia and eight in Peziza vesicidosa ; after brachymeiosis 
is complete there are two chromosomes in Otidea, four in Peziza. 
7. The spores are delimited to some extent by vacuoles, but mainly 
by the astral rays. It is suggested that these may represent the paths of 
activity of an enzyme generated at the centrosome and producing chemical 
changes in the surrounding cytoplasm. 
