Development of the Fetus: General; Month 1

Week One: The ovum is in the Fallopian tube; fertilization occurs in the upper tube then the ovum passes down the tube and usually reaches the uterus by the end of the week.

Ovum - Egg.

Week Two: The ovum rapidly increases in size and is fully imbedded in the decidua reflexa (not pictured) at the end of this period. An ovum believed to be in it's 13th day: There is no appearance of embryonic structure. While the ovum is covered in villi, the contact with it and the uterine wall are rare. In another ovum believed to be in it's 14th day: An amnion and allantois, the embryo, the medullary groove, and the heart had begun forming.

Decidua reflexa - The padding in the uterus formed in preparation for childbearing. 

Villi - Finger-like projections, increase surface area for better absorption of substances.

Amnion - Innermost membrane surrounding the embryo.

 Allantois - Membrane outside the amnion that grows into part of the placenta.

Medullary groove - I'll let you know when I know.

Week Three: The embryo has curved into a crescent moon shape. The beginnings of a spine, protovertebrae, are fully formed by the end of the week. The Nervous System: the primary divisions of the brain are visible and the primitive ocular and auditory vesicles are formed. Primary circulation is established; the umbilical cord is being formed between the alimentary canal and the yolk-sac; the branchial arches are formed; the limbs appear as short buds and the Wolffian Bodies are visible. 

Vesicle - A fluid or air-filled sac.

Ocular - Pertaining to the eye or the sense of sight.

Auditory - Pertaining to the sense of hearing.

Alimentary canal - Musculo-membranous tube, about 30 feet long, 

extends through the body from mouth to anus and covers the entire digestive tract.

Yolk-sac - Supplies nutrition to the embryo in early formation. (Ends about 5th-6th week)

Branchial arches - Essentially gill-like structures that form into structures of 

the face, chin, neck, and ears. 

Week Four: The umbilical tube is fully developed; the caudal extremity projects out. Upper and lower limbs appear as more than buds and the cloacal aperture appears. The heart separates into the right and left halves; ganglia and the anterior roots of the spinal nerves, the olfactory fossa, the lungs and the pancreas can be made out.

Caudal extremity - Essentially a tail.

Cloacal aperture - The secretion point.

Ganglia - Structure containing a collection of nerves.

Anterior roots - The front (anterior) base of the roots connected to the spinal cord.

Olfactory - Pertaining to the sense of smell.

Fossa - A shallow depression or hollow.