The pupil the latter, knowledge their union. That union takes place through the recitation of the Veda. So much with regard to knowledge. Next, with regard to offspring. The mother is the former element, the father the latter, offspring their union. That union takes place through procreation. So much with regard to offspring.

In plain words: The student is the second part, and knowledge is what joins teacher and student, through reciting the sacred texts. Mother and father are joined in their child, through having children.