Devayana Marga and Pitruyana Marga (देवयानमार्गः पितृयानमार्गः च।)

Devayana (Path to the Realm of Gods) and Pitrayana (Path to the Realm of Ancestors) is the journey to the higher realms. This can be explained with reference to the (Chandogya Upanishad) of the Sama Veda (5-3), where Shwetaketu, once came to the assembly of Panchalas, whose reigning monarch was Pravahana Jaivali. The King asks Shwetaketu five questions to gauge his understanding before imparting ब्रह्मविद्या (Brahma Vidya). These famous five questions are as below

1. From here (this लोक-loka) where do the people go (after death)?
2. How do the dead come back?
3. At what point do the paths of the देवयान (journey to the deva loka post death) and पित्रयान (journey to Pritr loka post death) get separated?
4. Why do fewer jivatmas attain पित्र् लोक (loka of ancestors)?
5. In Panchagni, पञ्चाग्नि the fifth ahuti (oblation), how does Apa Tattva get the name of Purusha?

Devayana (Path to the Realm of Gods)

The student who receives ब्रह्मविद्या with श्रद्धा (devotion) while doing तपस् in the forest can reach अर्चिर्देवाः (the God of Light) during Utkramana time. The student who has attained knowledge in the above-mentioned manner continues on his journey through the daytime, शुक्लपक्ष (full moon night) and when the Sun is in the Northern hemisphere. Then through the देवताः he reaches आदित्य, and from there to चन्द्र (the Moon God), moving further to the energy देव (उर्जा). There he is received by the देवदूत (Bhagawan’s assistants), who welcomes him to परमब्रह्म (the Absolute). This is देवयान मार्ग (the divine Way), the final destination in the Bhuloka dimension.

The human body is likened to the Cosmos. The Brahmajñāni (the wise one) goes to the higher realm through heart veins which are beyond hundred, unlike the ordinary person who transits through the veins lesser in number. A critical question that arises is how does he perceive through which veins he is passing through? Such discussion is unwarranted as one who has worshiped the Param purusha, is absorbed in the ultimate goal, the wise seeker who through his own education and by God’s grace being illuminated is able to recognize the veins that help him depart to a higher realm. Such an advanced soul can transit through the path of light and even if such a person dies at night or in Dakshinayana, the jnani will attain "brahmaLoka" (Moksha).

Pitrayana (Path to the Realm of Ancestors)

Another kind of life journey which involves discharging one’s functional responsibilities and fulfilling material desires with no interest in acquiring ब्रह्मविद्या and then eventually death. Such people during continue their journey at night time, during the कृष्णपक्ष (अमावास्या-dark moon) in the southern direction, reaching the धुमाभिमनि देवताः (wind gods) without seeing the संवत्सर देवताः they reach the पितृलोक (land of the ancestors) then to the Sky and eventually reach सोम (the grain of the Gods).

After their karma is exhausted, these souls come back to पृथ्वीलोक using the same path that they had used to go up, through the Sky, Wind, Smoke, Cloud, Rain. Determined by one’s कर्म, the soul is reborn embodied through अन्न (rice, wheat, herbs, tree, varieties of roots and medicinal herbs). By the power of धर्म and कर्म (दैव) and through these grains, one may be born either as a ब्राह्मण, क्षत्रिय or वैश्य. Such pious people are called as "रमनिया चरना." If however their accumulated sin during the human birth is high, then they could become dog, pig or a चान्डाल (outcaste). Those souls who do not traverse either of these paths, the देवयान and पितृयान, are reborn time and again in the lower species of life. Therefore पितृलोक pitri loka is not filled all the time.

This is also highlighted in the Vedavyasa Brahmasutras.

तदन्तरप्रतिपत्तौ रंहति सम्परिष्वक्तः प्रश्ननिरूपणाभ्याम् ( III-1-1)

Tadantarapratipattau raṁhati sampariṣvaktaḥ praśnanirūpaṇābhyām ( III-1-1)

When the transmigration of the soul takes place, the living being enters into the new body along with the subtle elements- manasa (mind), buddhi (intellect) and ahankara (ego). This is corroborated in the Upanishads. The Devayana and Pitrayana is described in the Brahmasutras in the 4th chapter, 2nd section.

The law of Creation is also explained in the  Bhagavad Gita (VIIth & VIIIth Chapters) and in the third chapter of Yajna chakra.