Conventionally, these joints at the flask necks are outer joints.

And while the soup was boiling they drew the flask necks out in a spitting blue flame until they were sealed shut.

Take one of your round flasks, put the yeast soup into it, then soften the glass of the flask neck in your blast lamp and draw the neck out and downward into a thin little tubeturn his little tube down the way a swan bends his neck when he's picking something out of the water.