Manava Dharmashastra 1.56

1.56

aṇvyaḥ sūkṣmā mātrā avayavā yasya so'ṇumātrikaḥ | puryaṣṭakamantarābhavadeho vā, svabhāvata eva vā''tmānaḥ sūkṣmāḥ | yathoktam | "sa eṣa ātmā'ntarhṛdaye'ṇīyānityādi |" bījaṃ śarīrotpattikāraṇam | sthāsnu vṛkṣādijanmahetubhūtam | cariṣṇu jaṅgamam | samāviśatyadhitiṣṭhati pratinibadhyate | yadā tena saṃsṛṣṭaḥ prāṇādibhistadā mūrtiṃ vimuñcatyābadhnāti śarīraṃ gṛhṇātītyarthaḥ |

When, invested with minute particles, the Individual enters the moveable or immoveable Seed;—then, becoming united with the aforesaid (Subtile body), it assumes the (new) body.—(56)

Medhātithi’s commentary (manubhāṣya):
‘Invested with minute particles’—that which is constituted by minute, subtile, particles, parts. This may refer either to the ‘Puryaṣṭaka,’ or the ‘Intermediate Body’; or to the Soul itself;—all Souls being by their very nature, subtile; as stated in such texts as ‘This Self within the heart, is extremely minute.’

‘Seed’—The source of the physical body;—‘Immoveable,’ that which is the cause of the birth of trees, &c.;—‘Moveable,’ animate.—‘Enters into,’ becomes enclosed in;—then ‘united with the aforesaid,’—i.e., with Prāṇa and other constituents of the subtile body)—then ‘it assumes,’ attaches to itself, the body; i.e., takes up the new corporeal frame.—(56)