| Term | Definition |
|---|
| Implantation | When blastocysts loosely attaches to the endometrium within the uterus, in the lower fundus or body; inner cell mass facing endometrium |
| Ectopic pregnancy | Development of an embryo outside the uterine cavity; usually caused by scarring (infection), decreased smooth muscle contractions, and abnormal anatomy |
| Second week of development | Development of the trophoblast, which becomes part of the fetal membrane called the chorion; development of the embryonic membranes - form outside of the embryo along with the chorion |
| Chorion | Forms the main structure of the embryonic placenta |
| Trophoblast | Secretes enzymes that allow deeper implanation of an embryo into the endometrium and human chorionic gonadotropin (hCG) |
| Human chorionic gonadotropin | Hormone that maintains the corpus luteum |
| Amnion | Thin protective membrane that surrounds the embryo as it grows; contains an amniotic cavity which fills with amniotic fluid |
| Amniotic fluid | Filtrate from the mothers blood and secretions from the fetus; regulates temperature of embryo and acts as a shock absorber |
| Yolk sac | Source of blood cells in an early fetus; shock absorber |
| Allantois | Very vascular pouch in the wall of the yolk sac; site of early blood cell formation and forms part of the umbilical cord |
| Third week of development | Includes gastrulation, formation of the neural tube, and formation of the placenta |
| Gastrulation | Development of a gastrula from a blastocyst; forms three primary layers that make up the tissue of a fetus; includes the endoderm, mesoderm, and ectoderm |
| Endoderm | Ultimately becomes the epithelial lining of all tracts and ducts of the body |
| Mesoderm | Gives rise to connective tissue, as well as muscle and bone |
| Ectoderm | Becomes the epidermis (skin) and the nervous system |
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