مِن نُطفَةٍ إِذا تُمنىٰ
From a drop of [seminal] fluid when emitted.
EXEGESIS
Nuṭfah means a limpid water, and has been used to refer to the seminal fluid of the male. The mixture of the male and female reproductive material was also called nuṭfah.
Tumnā (emitted) comes from manā which means measurement. The seminal fluid is called manī because it is measured by the knowledge of God. Tumnā is a passive verb signifying the male seminal fluid which is ejected with force, as an effusing fluid (86:6), by the measurement of God.
EXPOSITION
This verse explains how the male and female mentioned in the previous verse are created. It is indeed amazing that a complex multicellular human with all the various delicate organs is created from an admixture of just two cells. Both the male and the female emanate from this same source material. Even though they are different, the source is one. This is meant to cause one to reflect on all the earlier verses, and how what is mentioned in them emanates from one source.
The added description of when emitted is also worth noting. The potential of the seminal fluid is only achieved once it sets into motion and is paired with the female. On its own it is a base fluid (77:20), but when it travels the path intended for it and that it has been made for, it becomes lofty and laudable; He formed you and perfected your forms, and toward Him is the destination (64:3). The physical aspect of his creation can be considered an allegory for man and his spiritual journey as well. Only by traveling the path does he actualise his potential and what he was created for.
[1] Raghib, p. 811.
[2] Tibyan, 9/437.
[3] Tibyan, 9/437; Raghib, p. 779.
[4] Base (mahīn) means lowly and loathsome.