1Thus have I heard. On one occasion the Blessed One was dwelling at Rājagaha on Mount Inda's Peak, the haunt of the yakkha Indaka. [n.558] Spk: This was the yakkha who dwelt on Inda's Peak. Sometimes a peak is named after a yakkha, sometimes a yakkha after a peak. Then the yakkha Indaka approached the Blessed One and addressed him in verse:

2v.802 "As the Buddhas say that form is not the soul,
How then does one obtain this body?
From where do one's bones and liver come?
How is one begotten in the womb?" [n.559] Spk glosses sajjati in pāda d with laggati tiṭṭhati, "sticks, persists," apparently taking sajjati as equivalent to Skt sajyate (see MW, s.v. sañj (2)). But the word may be a passive representing Skt sṛjyati for which MW (s.v. sṛj) lists as meanings "to create, procreate, beget, produce." I translate on the assumption that this is the original derivation. See too PED, s.v. sajati (1).
Spk says that this yakkha was a personalist (puggalavadi) who held the view that a being is produced in the womb at a single stroke (ekappaharen’ eva satto matukucchismiṁ nibbattati). The Buddha’s answer is intended to refute the yakkha's belief by showing that a being develops gradually (anupubbena pana vaḍḍhati).

The Blessed One:

3v.803 "First there is the kalala;
From the kalala comes the abbuda;
From the abbuda the pesi is produced;
From the pesi the ghana arises;
From the ghana emerge the limbs,
The head-hair, body-hair, and nails.

4v.804 And whatever food the mother eats—
The meals and drink that she consumes—
By this the being there is maintained,
The person inside the mother's womb." [n.560] The Pali terms refer to the different stages in the formation of the embryo. Spk: The kalala is the size of a drop of oil placed on the tip of a thread made from three strands of wool. After a week from the kalala comes the abbuda, which is the colour of meat-washing water. After another week, from the abbuda the pesi is produced, which is similar to molten tin (Spk-pṭ: in shape, but in colour it is pink). After still another week, from the pesi the ghana arises, which has the shape of a chicken egg. In the fifth week, from the ghana emerge the limbs: five pimples appear, the rudiments of the arms, legs, and head. But the head-hairs, body-hairs, and nails are not produced until the forty-second week.

1Evaṁ me sutaṁ—​ ekaṁ samayaṁ bhagavā rājagahe vihārati indakūṭe pabbate, indakassa yakkhassa bhavane. Atha kho indako yakkho yena bhagavā tenupasaṅkami; upasaṅkamitvā bhagavantaṁ gāthāya ajjhabhāsi:

2"Rūpaṁ na jīvanti vadanti buddhā,
Kathaṁ nvayaṁ vindatimaṁ sarīraṁ;
Kutassa aṭṭhīyakapiṇḍameti,
Kathaṁ nvayaṁ sajjati gabbharasmin"ti.

3"Paṭhamaṁ kalalaṁ hoti,
kalalā hoti abbudaṁ;
Abbudā jāyate pesi,
pesi nibbattatī ghano;
Ghanā pasākhā jāyanti,
kesā lomā nakhāpi ca.

4Yañcassa bhuñjatī mātā,
annaṁ pānañca bhojanaṁ;
Tena so tattha yāpeti,
mātukucchigato naro"ti.