The outermost layer of a blastocyst is called?
a) ectoderm
b) mesoderm
c) endoderm
d) trophoblast

Options

A. ectoderm
B. mesoderm
C. endoderm
D. trophoblast

Explanation

To solve the question "The outermost layer of a blastocyst is called?", we can follow these steps:

1. **Understand the Structure of the Blastocyst**: A blastocyst is a structure formed in the early development of mammals, typically at the 64-cell stage. It consists of an outer layer and an inner cell mass.

2. **Identify the Layers of the Blastocyst**: The blastocyst has two main components:
- The outer layer, which is responsible for implantation into the uterine wall.
- The inner cell mass, which will develop into the embryo.

3. **Determine the Function of the Outer Layer**: The outer layer of the blastocyst plays a crucial role in the implantation process. It is involved in attaching the blastocyst to the endometrium (the lining of the uterus).

4. **Select the Correct Term for the Outer Layer**: The outer layer of the blastocyst is known as the trophoblast. This layer is specifically responsible for the attachment to the uterine wall.

5. **Evaluate the Options**: The options given are:
a) ectoderm
b) mesoderm
c) endoderm
d) trophoblast

The ectoderm, mesoderm, and endoderm are germ layers that develop later in embryonic development, not during the blastocyst stage.

6. **Conclude the Answer**: Based on the information, the correct answer is **d) trophoblast**.