ثُمَّ جَعَلْنَاهُ نُطْفَةًThereafter We made him a Nutfah.Here the pronoun refers back to humankind, as in another Ayah:الَّذِى أَحْسَنَ كُلَّ شَىْءٍ خَلَقَهُ وَبَدَأَ خَلْقَ الاِنْسَـنِ مِن طِينٍثُمَّ جَعَلَ نَسْلَهُ مِن سُلَلَةٍ مِّن مَّأءٍ مَّهِينٍand He began the creation of man from clay. Then He made his offspring from semen of despised water. (32:7-8)meaning, weak, as He says:أَلَمْ نَخْلُقكُّم مِّن مَّأءٍ مَّهِينٍفَجَعَلْنَـهُ فِى قَرَارٍ مَّكِينٍDid We not create you from a despised water, Then We placed it in a place of safety. (77:20-21) meaning the womb, which is prepared and readily equipped for that, فِي قَرَارٍ مَّكِينٍ... in a safe lodging.إِلَى قَدَرٍ مَّعْلُومٍفَقَدَرْنَا فَنِعْمَ الْقَـدِرُونَFor a known period. So We did measure; and We are the Best to measure. (77:22-23)meaning, for a known period of time, until it is established and moves from one stage to the next.Allah says here