Sometimes, using a heated propagator can lead to a reduced number of resulting seedlings, rather than an increased one.
This is how a heated prop works: It heats the soil to a temperature that ensures a seed will germinate. Some don't need heat [brassicas, lettuces for example - hate heated propagators in the UK - really there is no need]. So all you are doing is 'prodding' them with enough heat to kick them into action. Then you must remove them from the heat unless you are planning on growing them under heat for the duration of their life.
So when you see the above [or earlier if you can], [scuse the photo - it was on my phone but needed to be captured pronto] take them out of the heated prop! If you even leave them in for another day - you will end up with lanky soft seedlings, that can flop and die as soon as they experience any cool air or wind - and you set yourself up for stem rot on that long floppy sappy stem esp on cucurbits.
The same goes for any other new seedlings - as soon as you see some green - this means the heated prop has worked - so get those newly germinated seedlings out and into fresh air.
I see so many 'what has happened to my seedlings, they were in a heated prop for a week and now they are dead' posts - it really is a waste of time having one if it's not used properly. Also, seedlings can be set back hugely if they get too much heat at the start as they need more hardening off when you want them to go outside.
It's a heated PROPAGATOR so if you are using it to propagate seeds, whip them out once they are no longer seeds.
#they can also be used for cuttings, but this advice is for seeds only.
Oh, the above seeds were sown on Saturday, went into the prop yesterday and came out today. With squashes it can be that quick. Don't make me tell you again!