Okay, let me see if I got this. Soooooo... let's skip right down to the aftermath of sex. つ◕ل͜◕)つ Sperm is in the vagina, undergoes capacitation (becomes motile and active) due to the slightly alkaline environment of the vagina. Sperm travels up through the cervix into the uterus and into the fallopian tube. Meanwhile, the egg which has just been released from the ovary (due to LH surge) enters fallopian tube and is being pushed towards the sperm by the cilia in the tube (defects in cilia can cause ecotopic pregnancy). Only a few of millions of sperm make it to this point (makes you appreciate how lucky we are, lol). This is where things get a little rusty for me. So the sperm encounters the egg and unergoes the acrosome process, releasing hydrolytic enzymes that help it get into the egg follicle. Inside the follice, the sperm enters the egg and immediately after this occurs, two things happen: fast-and-slow block to polyspermy. I believe the fast block occurs due to slight depolarization of the egg membrane (due to calcium influx). This prevents some sperm from getting in. Slow block takes a few minutes longer and occurs due to intracellular release of calcium from egg's smooth ER. This causes the corticol reaction (sp?) which causes the release of vesicle containing enzymes and other stuff which totally prevents additional sperm from entering. Meanwhile, sperm entry causes the egg to increase it's metabolic activity. This allows the egg to continue the maturation process to become a fully mature egg by finishing meiosis II (pre-ovulation, it wsa stuck in prophase I of meiosis, post-ovulation, it matured to metaphse II of meiosis II with 3 polar bodies). Once meosis is complete, we have two nuclei: pronuclei (sperm) and pronuclei (egg); these fuse to form the zygote. Immediately, the zygote (still in fallopian tube) undergoes a series of rapid mitotic divisions (period without growth, only division) called cleavage: 2-4-6-8-16-32 cell (morula). These cells are totipotent (can form all cells). Morula continues cleavage to become Blastocyst. The blastocyst embeds itself into the uterine lining (this person is officially empregnated). Blastocyst continues divising to Gastrula (where germ layers form) --> Gastrula (with outer layer trophoblast - this forms: chorion and part of the placenta. (Chorion during first trimester (1-2months) releases hCG which helps maintain corpus luteum; The placenta receives nutrients, O2 from mom, and releases wastes). Anyways, back to the germ layers, these layers form the embryo and this embryo eventually develops into the fetus which gives rise to a new born baby at birth. I'm surprised I remember all that. Other things worth mentioning: identical and fraternal twins. Identical twins occurs when the early embryo divides in two, or if the zygote divides in two. Fraternal twins occurs by two eggs and two sperm (nonidentical DNA) and also it has a genetic component (identical twins do not).
I'm a little rusty on that myself, but you should know the embryonic origins of most germ layers. Seems like a popular question. My knowledge is very limited here, but based on what I can remember. Notocord formation occurs first (formed by the mesoderm). This somehow signals the formation of a neural groove (ectoderm origin) which eventually gives rise to the actual nervous tissue (so the nerve and associated nervous tissue is of ectoderm origin, not mesoderm). This phenomenon where a group of cells influence the formation of other cells is known as induction. The notocord breaks off and gives rise to the vertebrate (I think? not sure). You should be familiar with the terms determination and differentiation. Determination is when a cell is limited to develop within a given pathway (germ line, endoderm, mesoderm, ectoderm), whereas differentiation is the actual maturation of the cell to some specialized tissue. Embryonic Stem Cells are pluripotent (can give rise to all tissues except placenta) and they are derived from the inner cell mass (that eventually becomes germ layers) within the early stages of the gastrula.
