Vagina is acidic-can kill sperm. Prostate secretions in sperm neutralize acid. Proteins on sperm head find egg receptors. Produce sperm, but not able to trigger egg to allow entry. Implantation blastocyst reaches uterus and implants into uterine lining.
Cavities form to define the amnion and yolk sac.
Mesoderm forms muscle and connective tissue, and participates in the development of most organs. Endoderm gives rise to the gut, digestive organs and lungs. This is the first sign of an organ, long before heart, GI tract, muscles, or other organ systems form. There are currently more than 20 STDs identified, with millions of people being infected every year. STDs are caused by bacteria, protozoa, fungi, parasitic insects and viruses. .