俱舍論頌疏論本 9.40

9.40

前後際各二者，謂前後際各立二支，無明、行在前際，生、老死在後際。

"Two each in the prior and posterior periods" means that two links each are established in the prior and posterior periods: ignorance and karmic formations are in the prior period, while birth, and aging and death are in the posterior period.