AITAREYA UPANISHAD · 2.1.1

पुरुषे ह वा अयमादितो गर्भो भवति यदेतद् रेतः । तदेतत् सर्वेभ्योऽङ्गेभ्यस्तेजः संभूतमात्मन्येवात्मानं बिभर्ति तद्यदा स्त्रियां सिञ्चत्यथैनज्जनयति तदस्य प्रथमं जन्म ॥ १ ॥

puruṣe ha vā ayamādito garbho bhavati yadetad retaḥ । tadetat sarvebhyo’ṅgebhyastejaḥ saṃbhūtamātmanyevātmānaṃ bibharti tadyadā striyāṃ siñcatyathainajjanayati tadasya prathamaṃ janma ॥ 1 ॥

In the male first the unborn child becometh. This which is seed is the force and heat of him that from all parts of the creature draweth together for becoming; therefore he beareth himself in himself, and when he casteth it into the woman, ’tis himself he begetteth. And this is the first birth of the Spirit.

Sri Aurobindo · CWSA 18, p. 202