In MN12 that you referenced, the four types of yoni (ways of birth) are stated:
“Sāriputta, there are these four kinds of reproduction” (Catasso kho imā, sāriputta, yoniyo.)”
A better translation would be: “Sāriputta, there are these four kinds of births”
The sutta states those four kinds as, “Aṇḍajā yoni, jalābujā yoni, saṁsedajā yoni, opapātikā yoni” which can be translated as “Born from an egg, from a womb, from moisture, or spontaneously.”
In a way, ALL births (that take place at the cuti-patisandhi moment) are opapātika or spontaneous births.
– The “four modes of birth” apply to mainly humans and animals.
– The manomaya kaya/gandhabba created spontaneously by kammic energy can lead to “births with physical bodies” in the other three ways.
Thus, the “seed” for the other three modes comes from the manomaya kaya/gandhabba.
– For humans, a gandhabba gets into a womb and is born with a physical body. That is the “jalābujā yoni” or “birth from a womb” with “breaking out of the amniotic sac.”
– For animals, all three modes apply. Some (monkeys, deer, etc) are born like humans (jalābujā yoni“); Chicken, birds, etc. are born from an egg (aṇḍajā yoni). The last mode of saṁsedajā yoni is rare for big animals and applies mostly to smaller ones. Here the required “chemical base” for the gandhabba happens naturally in rotten meat or leaves.
– Note that in all three of those modes, a “seed” (manomaya kaya/gandhabba) must have been first produced by kammic energy.
Also, note that in all realms above the human realm, all births involve one step, i.e., spontaneous birth (opapātika) via kammic energy.