Semen is formed by desire. From desire is produced the menstrual flow. In the union of semen and blood, generated by Samana and Vyana, the pair that consists of Prana and Apana, enters, moving transversely and upwards. Vyana and Samana both form a pair which move transversely.
कामात् संजायते शुक्रं शुक्रात् संजायते रजः। समानव्यानजनिते सामान्ये शुक्रशोणिते॥ प्राणापानाविदं द्वन्द्वमवाक् चोर्ध्वं च गच्छतः। व्यान: समानश्चैवोभौ तिर्यग् द्वन्द्वत्वमुच्यते॥