Time Period

Major Events

Period of cleavage

First week

Cells undergo mitosis, blastocyst forms; inner cell mass appears; blastocyst implants in uterine wall Size: 1/4 inch (0.63 centimeters), weight: 1/120 ounce (0.21 grams)

Embryonic stage

Second through eighth week

Inner cell mass becomes embryonic disk; primary germ layers form, embryo proper becomes cylindrical; main internal organs and external body structures appear; placenta and umbilical cord form, embryo proper is suspended in amniotic fluid

Size: 1 inch (2.5 centimeters), weight: 1/30 ounce (0.8 grams)

Fetal stage

Ninth through twelfth week

Ossification centers appear in bones, sex organs differentiate, nerves and muscles coordinate so that the fetus can move its limbs

Size: 4 inches (10 centimeters), weight: 1 ounce (28 grams)

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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