## **Core Concept**
The zygote, formed by the union of sperm and egg, requires a source of nutrition to undergo cell divisions and development. In humans, the initial stages of development are supported by the nutrients stored within the egg and later by the implantation in the uterine wall.

## **Why the Correct Answer is Right**
The correct answer, **C. Uterine secretion (or more accurately, the implantation in the endometrium and subsequent support from the uterine lining)**, indicates that after the zygote undergoes several cleavages while traveling through the fallopian tube, it develops into a blastocyst. The blastocyst then implants into the endometrium (the lining of the uterus), where it becomes dependent on the maternal uterine secretions and the implantation site for its nutrition. This process involves complex interactions between the developing embryo and the maternal uterine environment.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because while the zygote initially may rely on stored nutrients from the egg (oocyte), it quickly outgrows these reserves and requires an external source for continued development.
- **Option B:** This option is incorrect as it does not directly relate to a known source of nutrition for the zygote/embryo during its early development stages.
- **Option D:** This option is incorrect because, although ovarian hormones play a crucial role in preparing the uterine lining for potential implantation, they are not the direct source of nutrition for the zygote.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that after implantation, the embryo derives its nutrition from the maternal circulation via the placenta. The process of implantation is critical and usually occurs 6-10 days post-fertilization. Failure of implantation can lead to failed pregnancy.

## **Correct Answer:** . Uterine secretion.