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In human female the blastocyst
A
Forms placenta even before implantation
B
Gets implanted into uterus 3 days after ovulation
C
Gets nutrition from uterine endometrial secretion only after implantation
D
Gets implanted in endometrium by the trophoblast cells
Detailed Solution
The blastocyst has an outer layer of cells called the trophoblast and an inner group of cells called the inner cell mass.
The trophoblast layer gets attached to the endometrium; the uterine cells then divide rapidly and cover the blastocyst, so that it becomes embedded in the endometrium. This is implantation.
Implantation occurs about 7 days after fertilisation, not 3 days after ovulation.
The placenta is formed only after implantation, from the chorionic villi and uterine tissue.
Even before implantation the blastocyst is nourished by uterine secretions, so it does not get this nutrition 'only after' implantation.
Hence the blastocyst gets implanted in the endometrium by the trophoblast cells.
The trophoblast layer gets attached to the endometrium; the uterine cells then divide rapidly and cover the blastocyst, so that it becomes embedded in the endometrium. This is implantation.
Implantation occurs about 7 days after fertilisation, not 3 days after ovulation.
The placenta is formed only after implantation, from the chorionic villi and uterine tissue.
Even before implantation the blastocyst is nourished by uterine secretions, so it does not get this nutrition 'only after' implantation.
Hence the blastocyst gets implanted in the endometrium by the trophoblast cells.