The Alpha and Beta are incubated and the Gamma, Delta, and Epsilon undergo the Bokanovsky Process. Accroding to Enotes:
This causes the fertilized egg to bud and create up to 96 identical individuals ...The bottles are placed on assembly lines, which move them along at a calculated rate. Thus, they reach the Decanting Room nine months, or 267 days, after fertilization, at a rate of eight meters a day.
In the incubators, the Alpha and Beta test-tubes are stored in an insulated room at 35 degrees Centigrade:
And opening an insulated door he showed them racks upon racks of numbered test-tubes. "The week's supply of ova. Kept," he explained, "at blood heat; whereas the male gametes," and here he opened another door, "they have to be kept at thirty-five instead of thirty-seven. Full blood heat sterilizes." Rams wrapped in theremogene beget no lambs.
Some of the other bottles (for the lower castes) undergo bokanovskification:
On a very slowly moving band a rack-full of test-tubes was entering a large metal box, another, rack-full was emerging. Machinery faintly purred. It took eight minutes for the tubes to go through, he told them. Eight minutes of hard X-rays being about as much as an egg can stand. A few died; of the rest, the least susceptible divided into two; most put out four buds; some eight; all were returned to the incubators, where the buds began to develop; then, after two days, were suddenly chilled, chilled and checked. Two, four, eight, the buds in their turn budded; and having budded were dosed almost to death with alcohol; consequently burgeoned again and having budded–bud out of bud out of bud–were thereafter–further arrest being generally fatal–left to develop in peace. By which time the original egg was in a fair way to becoming anything from eight to ninety-six embryos– a prodigious improvement, you will agree, on nature. Identical twins–but not in piddling twos and threes as in the old viviparous days, when an egg would sometimes accidentally divide; actually by dozens, by scores at a time.
I think that you are referring to things that the book says in Chapter 1 about the conditions in the place where the bottles are.
We are told that the temperature is "tropical" and that they have to be somewhere where there is no light.
The reason for this is due to what is in the bottles. After all, inside the bottles are human fetuses. Because of that, the bottles have to be in a place that is quite a bit like an actual human uterus. So that means it has to be quite warm and it has to be dark.