In the following week, if it takes a week for these daughter culture vessels to become what we call 'confluent', which means they've covered the whole floor and have stopped dividing. In the second week, you can now make four vessels and then the next week eight vessels and then 16, 32, 64, in each subsequent week. Well, if you do the math, you can easily see that within a few weeks, you're going to be overrun with bottles for which you have no real use, since most experiments don't involve thousands of bottles. What you can do with the surplus cells each week is freeze them and that's very important. You can now... there are techniques for freezing cells in a viable or living state, so that you can recover them or resurrect them months, years, decades later and that's an essential point. That's called 'cryobiology', or 'cryogenics'.

So that technology of freezing cells crept into the field actually from veterinary medicine, where the exact technique used in cell culture was lifted over from veterinary practice in circumstances of artificial insemination of cattle in the field. In fact, the preservation under cryogenic conditions is usually best done in liquid nitrogen, which is extremely cold, about -196 degrees Celsius, which is equivalent to putting your finger, if you were foolish enough to do that, into a fire. It's that cold. You get the same disastrous results. And so that technology was just beginning to enter the field about this time and it exists even to this day with very little change. You can also preserve the cells at a temperature of dry ice, which is much warmer, it's about -70 degrees Celsius. And you can also, of course, preserve them at that temperature in an electrically driven freezer. So you have those three options. Liquid nitrogen is best because there are no moving parts. You top up the liquid nitrogen every three or four weeks and you don't have to worry too much about failure, although it does happen occasionally.