The first step of mitosis is:

 # The first step of mitosis is which of the following? 

a. Telophase

b. Metaphase

c. Prophase

d. Anaphase 

The correct answer is: C. Prophase.

Prophase is the first step of mitosis in which four structural changes occur.

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Melanoblastoma are derived from:

 # Melanoblastoma are derived from:

A. Basal epidermal cells

B. Neural crest cells

C. Prickle cells of epidermis

D. Somato pleuric mesoderm

The correct answer is B. Neural crest cells.

Melanoblastoma are derived from Neural crest cells.

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Notochord develops in:

 # Notochord develops in:

A. 3rd week

B. 3rd month

C. 6th month

D. 10th month

The correct answer is A. 3rd week.

The notochord arises from axial mesoderm at about 16 days and is completely formed by the beginning of the fourth week. It defines the longitudinal axis of the embryo, determines the orientation of the vertebral column, and persists as the nucleus pulposus of the intervertebral discs.

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Implantation occurs at the stage of:

 # Implantation occurs at the stage of:

A. Zygote

B. Morula

C. Blastocyst

D. Primary villi

The correct answer is C. Blastocyst.

The blastocyst is a structure formed in the early development of mammals. It possesses an inner cell mass (ICM) which subsequently forms the embryo. The outer layer of the blastocyst consists of cells collectively called the trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoel. The trophoblast gives rise to the chorion and amnion that surround the embryo.

In humans, blastocyst formation begins about 5 days after fertilization when a fluid-filled cavity opens up in the morula, the early embryonic stage of a ball of 16 cells. The blastocyst has a diameter of about 0.1–0.2 mm and comprises 200–300 cells following rapid cleavage (cell division). About seven days after fertilization, the blastocyst undergoes implantation, embedding into the endometrium of the uterine wall. There it will undergo further developmental processes, including gastrulation. Embedding of the blastocyst into the endometrium requires that it hatches from the zona pellucida, which prevents adherence to the fallopian tube as the pre-embryo makes its way to the uterus.

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