This is the way I understand the post-conception waltz:
Sperm enters egg.
One half of the egg is considered “animal” (animated) and becomes the new creature while the other half is “vegetal” and becomes the support system, the enveloping tissues and the placenta attached to the inner surface of the uterus.
(A zygote is called an embryo after it attaches to the uterus lining.)
The point where the sperm enters the egg becomes one pole of the organization of the zygote (fertilized egg) and the point opposite is then the other pole. In vertebrates, this line through the zygote is where the spine develops. So vertebrates are almost always bilateral, two equal sides.
The animal part develops into three primal or potential groups of cells.
"In general, ectoderm develops into parts of the skin, the brain and the nervous system. Mesoderm gives rise to bones, muscles, the heart and circulatory system, and internal sex organs. Endoderm turns into the inner lining of some systems, and some organs such as the liver and pancreas."
“Neural crest cells are a temporary group of cells unique to vertebrates that arise from the embryonic ectoderm cell layer, and in turn give rise to a diverse cell lineage—including melanocytes, craniofacial cartilage and bone, smooth muscle, peripheral and enteric neurons and glia.”
The surface ectoderm develops into: epidermis, hair, nails, lens of the eye, sebaceous glands, cornea, tooth enamel, the epithelium of the mouth and nose. The neural crest of the ectoderm develops into: peripheral nervous system, adrenal medulla, melanocytes, facial cartilage, dentin of teeth."
"By the 17th-20th day, the neural plate is formed. The neural plate turns into the neural groove on day 23. Two days later, the neural groove curls, which then turns into the neural tube, which marks the beginning of brain development. "
"The neural tube forms the eventual central nervous system, with the bottom becoming the spine, and the top becoming the brain. The 7th week marks the formation of the genitals, which decides the baby’s gender. By the 8th week of development, the baby is no longer an embryo, and formally becomes a fetus. Additionally, all of the baby’s organs are formed by this time."
"Neurulation refers to the folding process in vertebrate embryos, which includes the transformation of the neural plate into the neural tube. The embryo at this stage is termed the neurula."
Somewhere along in these days is when the difference between “tame” and “wild” is defined for the body. It might be actual nuclear genome change from the egg, or it might be epigenomic, a change added to the genome by methylation (a molecular switch) which can be imposed by the environment, both that inside the womb or outside after birth. Presumably, change could also affect the mother who IS the environment for the fetus. If something happened to the mother during the first two months, when she barely realizes she is pregnant, it will affect these steps of development. Tame and wild are predilictions, inclinations, that can be influenced by environmental conditioning or epigenomic modulation.