In human female the blastocyst :-

Options

A. Gets implanted into uterus 3 days after ovulation
B. Gets nutrition from uterine endometrial secretion only after implantation
C. Gets implanted in endometrium by the trophoblast cells
D. Forms placenta even before implanation

Answer

### Step-by-Step Solution:

1. **Definition of Blastocyst**: The blastocyst is an early stage of embryonic development that occurs after fertilization and several rounds of cell division. It typically consists of 50 to 100 cells.

**Hint**: Remember that the blastocyst is a crucial stage in human development that follows fertilization.

2. **Timing of Implantation**: The blastocyst gets implanted into the uterus approximately 3 days after ovulation, although the entire implantation process can take about a week to complete.

**Hint**: Keep track of the timeline from ovulation to implantation; it helps understand the development stages.

3. **Nutritional Source**: After implantation, the blastocyst receives nutrition from the uterine endometrial secretions. Before implantation, it relies on the nutrients available in the uterine environment.

**Hint**: Think about how the blastocyst sustains itself before and after it attaches to the uterine wall.

4. **Structure of the Blastocyst**: The blastocyst consists of two main parts: the outer layer called the trophoblast, which will eventually help form the placenta, and the inner cell mass, which will develop into the embryo.

**Hint**: Visualize the blastocyst as having an outer layer and an inner mass; this distinction is important for understanding its function.

5. **Implantation Process**: The trophoblast cells play a critical role in the implantation process by attaching to the endometrial lining of the uterus.

**Hint**: Focus on the role of trophoblast cells in the implantation process; they are essential for establishing the connection with the mother's body.

6. **Formation of Placenta**: The placenta does not form before implantation. It develops later, typically after the embryo has established itself in the uterine lining, around 2 to 3 months into the pregnancy.

**Hint**: Remember that the placenta is a later development and is crucial for supporting the fetus as it grows.

### Summary:
In summary, the blastocyst is a crucial stage in human development that implants into the uterus, receives nutrition from the endometrial secretions, and consists of a trophoblast layer and an inner cell mass. The placenta forms after implantation, not before.