वयस्यासु मूर्ध्नो मतुप्

The word वयस्या means the bricks, the Upadhana mantras of which contain the word वयस् ॥ The affix मतुप् debars यत् ॥ A mantra which contains both the word वयस् and मूर्धन्, that Mantra is both वयस्वान् and मूर्धन्वान् ॥ Now in denoting sacrificial bricks put up with such a mantra, the affix यत् would have come by 4.4.125 after both these words वयस्वान् and मूर्द्धन्वान् ॥ The present sutra ordains मतुप् after मूर्द्धन्वान् ॥ Thus मूर्द्धन्व꣡तीरुपदधाति 'he collects Murdhanvati bricks'. The words वयस्याः and मूर्द्धन्वत्यः denote the same object. See (VI. I. 176).

Why do we say वयस्यासु ? The affix 'matup' will not be added to 'murdhan', if the Mantra contains only the word 'murdhan' and not 'vayas' also. In that case मूर्धन्याः will be the form, meaning 'bricks collected with the Mantra containing the word मूर्धन्', while मूर्धन्वत्यः means 'bricks collected with the Mantra containing the words murdhan and vayas'.

The sutra contains the word मूर्ध्नः ablative singular of मूर्धन् ॥ It ought to have been मूर्धन्वतः, for the affix matup is added to मूर्धन्वत्, the यत् being elided then by 4.4.125. The author has mentally elided the affix वत्, and shown in the aphorism the last stage of the word to which मतुप् is to be added.,