are formed in fetal life at specific locations in oral cavity through the growth of a bud of oral epithelium into underlying mesenchyme.
Primordia for parotid &sub mand. appears around 6th week
For sub lingual around 7-8 wk fetal life.
This epithelial bud grows into extensively to the branching solid cords&gradually develop a lumen & then becomes ducts
after this scretory portion develop.
Hope this helps