252 
C. W. EDGERTON 
develop into a single colony. The presence of one or both strains 
could usually be told by looking at the colony; but, to make absolutely 
sure, several times they were reisolated by dilution cultures. 
Over forty different asci were transferred, but the ascospores in 
some failed to germinate and some of the plates were so badly con- 
taminated with bacteria that they had to be discarded. From twenty 
one colonies that did develop from single asci, twelve contained both 
the plus and minus strains and nine contained only the minus strain. 
In Plate II, figure 2, is shown a petri dish in which all of the colonies 
developed from spores that formed in one of the colonies from a single 
ascus. Both plus and minus strains are present. This shows that 
not only were there both strains present in a single perithecium but 
that they were also present in a single ascus, and this would seem to 
prove conclusively that there was an actual fertilization between the 
two strains on the boundary line where they come together in a plate, 
that one strain furnished the antheridia and the other the oogonia. 
All the asci did not produce both the strains but this was hardly 
to be expected. The culture medium used was oat juice agar, and, 
as has been noted, some of the perithecia of the minus strain mature 
on this medium and the asci in these as a rule do not break up as 
readily as the well developed asci that normally form in the boundary 
line, and it is possible that some of these might have been transferred. 
The perithecia of the minus strain develop abundantly all over the 
surface of the colony and it would be possible to get some of them in a 
mount made from the boundary. The plus strain only develops in 
nodules and the chances would not be good for transferring any of 
these to a mount, and as can be seen there were none of the asci that 
developed the plus strain alone. Furthermore it is possible that some 
of the asci that were transferred were not mature. Immature asci 
are always more abundant in a mount than mature ones on account of 
the disintegration of the latter at maturity. Whether the immature 
ascus as a whole would germinate has not been determined, but if it 
would, the resulting colony would probably be of the minus strain as 
this is probably the strain that produces the oogonia. And then it is 
possible that some of the ascospores in an ascus would get started before 
the others and prevent the latter from developing to any extent. 
However, the presence of both strains in any of the asci demonstrates 
the fact that there is really a fertilization between the colonies and 
that the plus and minus strains should be classified as sexual strains. 
