After fertilization and the formation of a zygote, the developing organism undergoes several rounds of cell division known as cleavage. During this period the cells divide but do not grow in size, resulting in a dense cell mass. The morula is formed during this portion of development.

Following cleavage, the cell develops an inner cell mass and trophoblast, or outer layer, before implantation into the uterus. At this stage, the organism is considered a blastocyst.

Soon after implantation, cells begin to differentiate into the three germ layers through a process known as gastrulation. The resulting organism is considered a gastrula.

Gastrulation is quickly followed by development of the neural crest and preliminary nervous tissue, known as neurulation. The organism is then considered a neurula.

Note that the organism can be considered an embryo at any point from the first cleavage division to approximately the eighth week of development.

In order for the testes to develop, the only thing necessary is the SRY gene product, which is created from the SRY gene on the Y chromosome. DHT are involved in the development of other male sex traits during early development, such as the penis, while testosterone is involved in both early development and the development of secondary sexual traits during puberty. Wolfian excitatory hormone does not actually exist. Estrogen is not involved in the development of male sex organs.

Following gastrulation, three primary germ layers form: the ectoderm, the mesoderm, and the endoderm. We can typically predict which structures will form from the germ layers based on the layers of the body. The endoderm will form the digestive tract, the mesoderm will form the skeleton and muscles, and the ectoderm will form the skin and nervous system.

Because the fetal lungs and liver are not fully functional until birth, the fetal circulation uses structures in order to bypass these organs.

The ductus venosus is used to reroute blood from the fetal liver and deposit it into the right atrium.

The ductus arteriosus connects the fetal pulmonary arteries to the aorta to bypass the lungs, while the foramen ovale connects the right and left atria to reduce flow to the pulmonary arteries.

The umbilical vein carries blood from the uterus, where it have been loaded with oxygen from maternal circulation, back to the body.

The ductus arteriosus is used to reroute blood from the pulmonary trunk and place it into the aorta of the fetus, preventing flow to the non-functional fetal lungs. After birth, this structure deteriorates and closes. The ligamentum arteriosum is the vestigial structure that is seen in adults. Instead of connecting the pulmonary arteries and aorta, it helps to stabilize these structures.

The foramen ovale is a fetal window between the right and left atria to help blood bypass the pulmonary circuit. After birth, this window closes to become the fossa ovalis.