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.
This is the first sign of an organ, long before heart, GI tract, muscles, or other organ systems form. Folding So far, embryo has been living on stored energy in egg. Now, it begins to feed on mother's tissues through placenta. At end of 8 weeks, embryo is about 1 inch in length, weighs 1 gram, all organs are already visible. Heart is the size of a poppy seed, and has begun to beat.