Reincarnation is a fact: when the soul enters the microscopic body in the womb

According to the Srimad Bhagavatam and Vedic literature, the soul enters the womb at the time of conception. Specifically:

The soul enters the womb through the particle of male semen when it mixes with the female ovum. This is described in Srimad Bhagavatam 3.31.1:

sri-bhagavan uvaca

karmana daiva-netrena jantur dehopapattaye

striyah pravista udaram pumso retah-kanasrayah

"Under the supervision of the Supreme Lord and according to the result of his work, the living entity, the soul, is made to enter into the womb of a woman through the particle of male semen to assume a particular type of body."

The soul is present from the very beginning of conception, when the sperm and ovum mix. The Bhagavatam states:

kalalam tv eka-ratrena panca-ratrena budbudam

dasahena tu karkandhuh pesy andam va tatah param

"On the first night, the sperm and ovum mix, and on the fifth night the mixture ferments into a bubble."

1. The soul enters based on its past karma and under the supervision of the Supreme Lord, to obtain a particular type of body suited for its next life.
2. Consciousness develops gradually as the fetus grows. Initially there is no consciousness, similar to deep sleep. As the body develops and the nine openings (eyes, ears, nostrils, etc.) form, consciousness returns.
3. Once consciousness returns, the soul begins to experience sensations and emotions within the womb.

In the Mārkandeya Purana, one of the 18 Puranas or Vedic texts (Ch.11):

"The son said: - As soon as the male seed is mixed with female blood one, released from heaven or hell, enters into it.

(1) O father, the two kinds of seed being influenced by him he attains stability. He then grows into protoplasm, next into a bubble and then into a lump of flesh.

(2) The germ that grows up in the lumps of flesh called Ankura and then are gradually produced the five limbs.

(3) Then the minor limbs, fingers, eyes, nose, face, and ears are developed from (principal) limbs and from them the nails, &c.

(4) Then hairs grow on the skin and then those on the head. Thus does the embryo grow up along with the uterus.

(5) As a coconut fruit grows along with its case so does this increase along with its case, with is face bent downwards.

(6) It grows keeping its hands downwards to its thighs and sides; the thumbs are placed on the thighs and the other fingers before them.

(7) The eyes are behind the thighs and the nose is within the thighs. The hips are between the two heels; the arms and legs remain outside.

(8) Thus a Creature, lying in the womb of a female, grows up gradually; the embryos of other creatures lie in the womb according to their forms.

(9) It gets hardened by fire and lives by what is eaten and drunk; the embryo exists in the womb depending upon virtue and vice.

(10) The entrail called āpyāyanī fixed to its navel is attached to the entrail of the female and it grows there.

(11) Having its body nourished while in the womb, by the food and drink a creature gradually grows up.

(12) It then gets the recollection of its many births and then pushed hither and thither it comes to entertain a distaste (for such a state).

(13) Having been released from the womb - "I shall never do it again - I shall so strive that I shall not have to enter into the womb any more" - thus does it think remembering a hundred miseries of births originating from destiny which he had experienced before.

(14-15) Then in the course of time, the creature, with its face bent downwards, turns itself and is then born in the ninth or the tenth month.

(16) And coming out it is assailed by the Prajāpatya wind and tormented by the grief that is in its heart it bewails.

(17) Coming out of the womb it falls into an unbearable trance; it regains its consciousness when it feels the (surrounding) air.

(18) Then the enchanting illusion of Vishnu takes possession of it; having its soul possessed by it, it sustains a bewilderment of sense.

(19) With the loss of sense the creature comes of infancy, boyhood, youth and old age.

(20) A soul repeatedly goes through a cycle of births and deaths. In this way, he rolls like a clock on the wheel of the world." [1]

Western History:

Epigenesis was originally proposed 2,000 years earlier by Aristotle. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favoured explanation among embryologists.

1. Origin of epigenesis:Aristotle is indeed credited with first proposing the concept of epigenesis around 350 BC in his book "On the Generation of Animals"
2. Definition of epigenesis:Aristotle's concept of epigenesis proposed that the form of an animal emerges gradually from an unformed or relatively formless egg
3. Acceptance of epigenesis:The debate between epigenesis and preformationism continued for centuries
4. Displacement of preformationism:As a result of these observations, epigenesis gradually displaced preformationism as the favored explanation among embryologists

It's worth noting that while Aristotle is often called the "father" of epigenesis, some scholars argue that this is not entirely accurate in a literal sense, as the formal debate using these terms came much later. However, Aristotle's ideas were certainly influential and can be considered a precursor to the modern concept of epigenesis.

Conclusion:

So both in the west and in the east the embryology was known much before invention of Ultrasound and X-ray technology. According to the Bharat Vedic sources, the soul enters at the moment of conception through the male semen, but full consciousness develops gradually as the fetus grows over the course of the pregnancy. The soul's entrance is directed by karma and divine arrangement to facilitate its next incarnation.