The Talmud describes fetal formation and assigns the kidneys a role in bodily development, including the formulation that the Holy One places the kidneys in the fetus and gives it understanding. It also connects the kidneys with counsel and instruction.
תָּנוּ רַבָּנַן: שְׁלֹשָׁה שׁוּתָּפִין יֵשׁ בָּאָדָם — הַקָּדוֹשׁ בָּרוּךְ הוּא, וְאָבִיו, וְאִמּוֹ. אָבִיו מַזְרִיעַ הַלּוֹבֶן, שֶׁמִּמֶּנּוּ עֲצָמוֹת, וְגִידִים, וְצִפׇּרְנַיִם, וּמוֹחַ שֶׁבְּרֹאשׁוֹ, וְלוֹבֶן שֶׁבָּעַיִן. אִמּוֹ מַזְרַעַת אוֹדֶם, שֶׁמִּמֶּנּוּ עוֹר, וּבָשָׂר, וּשְׂעָרוֹת, וְשָׁחוֹר שֶׁבָּעַיִן. וְהַקָּדוֹשׁ בָּרוּךְ הוּא נוֹתֵן בּוֹ רוּחַ, וּנְשָׁמָה, וּקְלַסְתֵּר פָּנִים, וּרְאִיַּית הָעַיִן, וּשְׁמִיעַת הָאוֹזֶן, וְדִבּוּר פֶּה, וְהִלּוּךְ רַגְלַיִם, וּבִינָה, וְהַשְׂכֵּל. וְכֵיוָן שֶׁהִגִּיעַ זְמַנּוֹ לְהִפָּטֵר מִן הָעוֹלָם, הַקָּדוֹשׁ בָּרוּךְ הוּא נוֹטֵל חֶלְקוֹ וְחֵלֶק אָבִיו וְאִמּוֹ, מַנִּיחַ לִפְנֵיהֶם. אָמַר רַב פָּפָּא: הַיְינוּ דְּאָמְרִי אִינָשֵׁי ״פּוּץ מִלְחָא וּשְׁדִי בִּשְׂרָא לְכַלְבָּא״.
§ The Sages taught: There are three partners in the creation of a person: The Holy One, Blessed be He, and his father, and his mother. His father emits the white seed, from which the following body parts are formed: The bones, the sinews, the nails, the brain that is in its head, and the white of the eye. His mother emits red seed, from which are formed the skin, the flesh, the hair, and the black of the eye. And the Holy One, Blessed be He, inserts into him a spirit, a soul, his countenance [ukelaster], eyesight, hearing of the ear, the capability of speech of the mouth, the capability of walking with the legs, understanding, and wisdom. And when a person’s time to depart from the world arrives, the Holy One, Blessed be He, retrieves His part, and He leaves the part of the person’s father and mother before them. Rav Pappa said: This is in accordance with the adage that people say: Remove the salt from a piece of meat, and you may then toss the meat to a dog, as it has become worthless.