According to MN 12 there are four kinds of regeneration(Mahāsīhanāda Sutta MN 12).
From those four Aṇḍajā (from an egg), jalābujā (from womb) makes no issue. Everyone has an understanding about those two. When it comes to the last type, spontaneous (opapātikā) we could consider it as something that cannot be known through our (bare) senses and should be known using supernatural powers (dibbacakkhu). Gods, hell-beings, certain humans, and certain beings in the lower realms are born spontaneously. This one is important due to the fact that It is a wrong view to think: there are no beings that are born spontaneously (natthi sattā opapātikā).
Catasso kho imā, sāriputta, yoniyo. Katamā catasso? Aṇḍajā yoni, jalābujā yoni, saṃsedajā yoni, opapātikā yoni.
Sāriputta, there are these four kinds of reproduction. What four? Reproduction for creatures born from an egg, from a womb, from moisture, or spontaneously (MN12; Ven. Sujato tranl.).
Now the most conflicting part: regeneration from moisture (saṃsedajā yoni) which has a definition similar to that of the spontaneous generation.
There are beings who are born in a rotten fish, in a rotten corpse, in rotten dough, in a cesspool or a sump. This is called reproduction from moisture (Ven.Sujato tranl.).
However, this long debated problem of Spontaneous generation was ended with the presented evidence by several scientists, Redi, 1968; Needham, 1745; Spallanzani, 1765; and finally Louis Pasteur 1861.
See for more information (Is Spontaneous Generation Real?)
Now we are in a big trouble due to below part from the sutta MN 12.
“Sāriputta, when I know and see thus, should anyone say of me:
"The recluse Gotama does not have any superhuman states, any distinction in knowledge and vision worthy of the noble ones. The recluse Gotama teaches a Dhamma merely hammered out by reasoning, following his own line of inquiry as it occurs to him"
—unless he abandons that assertion and that state of mind and relinquishes that view, then as surely as if he had been carried off and put there he will wind up in hell (MN 12; Ven. Bodhi tranl.).
I think having doubt is not a problem but it would later cause trouble. So we should clear our doubts.
Having faith (saddhā) on the Buddha, we could come up with few theories (arguments?) to justify saṃsedajā yoni. (For the record, these are stated without the uncertainty.)
- Insect eggs were not considered as Aṇḍā (eggs) with a point of view that a egg is having hard outer shell (common terminology at the time).
These two types mainly can be separated with the difference between the protective coverings, which may be a single membrane in some invertebrates and be a tough outer shell (of many vertibrate eggs such as birds, reptiles).
And most of the eggs to be considered under the catagory are not visible to the naked eye. This might be the second reason not to take those as eggs.
Some beings in ghost realm (Petās) can be born from moisture as they consumes things like saliva, feaces, dead bodies, etc.
The beings mentioned here are unicellular organisms such as Bacteria, Amoeba, Paramecium
They show asexual reproduction mostly with sexual reproduction where both produce zygotes and ultimately spores (sexual or asexual) as the reproduction unit.
However then there is another problem them being sentient. Is it possible?
They might not be having sensory organs which keeps us in doubt about them having āyatanas and so called sentience (life). To identify as a sentient being they should have nerval system. That is the foremost thing that keeps them reacting and responding fast enough to identify them as sentient beings. In fact there is no other important factor that makes living forms Animal-like.
However, they have different ways of signal transductions using secondary metabolites. It is hard to send signals efficiently due to their huge body size in larger animals. Therefore they need extra support system like nerves. However, this similarity alone does not give enough support to make them much closer to animals than plants. Buddhism does not take plants as living beings but Jains (ekajīvī).
Then the last one I could come up with at the moment. There may be other possibilities as well.
- There were only three generally identified ways of regeneration at the time of the Buddha, but the Buddha introduced the last one which might need supernatural powers to know; opapātikā.
This can be supported by ten fold wrong views.
There is a wrong view: natthi sattā opapātikā (there are no being that are born spontaneously.)( MN 114 and many other suttas).
Explaining the science behind the story is not easy, even if the Buddha did, it would sound like nonsense to the people at the time with no advanced scientific knowledge. I think that might be the reason this was not explained further. Eventhough, the Buddha said that the beings are born in such and such places, there is no explainations that suggest the origin of their material part is from the meat or cloths or something like that as in the spontaneous generation.
This final point can be changed like this,
People at the Buddha’s time knew about these four types of reproduction. The Buddha adopted that view with all those ten factors about secular right view from the common terminology.
Don’t insist on local terminology and don’t override normal usage.’ That’s what I said, but why did I say it? And how do you insist on local terminology and override normal usage? It’s when in different localities the same thing is known as a ‘plate’, a ‘bowl’, a ‘cup’, a ‘dish’, a ‘basin’, a ‘tureen’, or a ‘porringer’. And however it is known in those various localities, you speak accordingly, obstinately sticking to that and insisting: ‘This is the only truth, other ideas are silly.’ That’s how you insist on local terminology and override normal usage.
And how do you not insist on local terminology and not override normal usage? It’s when in different localities the same thing is known as a ‘plate’, a ‘bowl’, a ‘cup’, a ‘dish’, a ‘basin’, a ‘tureen’, or a ‘porringer’. And however it is known in those various localities, you speak accordingly, thinking: ‘It seems that the venerables are referring to this.’ That’s how you don’t insist on local terminology and don’t override normal usage. ‘Don’t insist on local terminology and don’t override normal usage.’ That’s what I said, and this is why I said it (MN 139)
As Buddha stated in Arana
The buddha did not say there are four types of regeneration of beings just becase he heard common ideology. He says it by knowing the facts, that there really are four.