 Congratulations. You have a zygote. Okay, so look at this thing. It's giant. It was this huge, massive oocyte that then you added just some more DNA. And I'm going to tell you how massive it was. The zygote immediately starts going through mitosis, which makes perfect sense because we're going to go through mitosis to grow and divide. The zygote, unique DNA, half from mom, half from dad. Now we've got this unique person. And every single one of their cells from now on is going to contain that exact same set of DNA. So they go through mitosis, the zygote does, and becomes an embryo. And there's like, oh my gosh, someday I really want to take an embryology course and learn all the stuff that happens. I know that that would be so cool. Yeah, that's not going to happen right now. But there's all these different stages. It takes the little zygote, which isn't a zygote once it starts dividing. It takes it about four or five days to bounce down the fallopian tubes. So you would think it'd just be a straight shot. I mean, think about how long it took the sperm to get up there. They can swim, so that helps. While in the fallopian tubes, the fallopian tubes make food to feed the zygote, because the little guy might be hungry, right? And so it secretes stuff out there. And the zygote's like, but it's an embryo now. It's like, thank you, fallopian tube. It's like fallopian tube milk. It's not really called fallopian tube milk. It's just a substance produced by the fallopian tube to feed this little thing that is now bouncing down the fallopian tube toward the uterus. It keeps bouncing. Four or five days later, it arrives in the uterus. It's actually the same size that it was when it left the fallopian tube or when it started, because that oocyte was so huge that nuclear division is taking place and we're making a whole bunch of new cells, but we are just splitting the cytoplasm. There's no growth happening yet. Once the embryo reaches the uterus, now it's going to float around in the uterus for about five days. Before it finally finds the perfect place, I don't know. Like, this is the softest, squishiest, nicest place. This is the place with the best closest access to the best garden. I don't know. How do they decide where they want to dig in? And then they literally dig in and digest and eat the lining of your uterus. Remember, the lining of your uterus was all built up nice and fluffy and puffy thanks to progesterone and estrogen. So they both work together. And remember, I told you that it's full of like fat protein that is there for your embryo to eat. So it's just like yumptializing your uterine lining. Wow, that's crazy talk. While it's there, okay, are you worried about anything? Perhaps you should be worried about the fact that who's keeping the uterine lining there? Who's doing it? I'm going to give you a hint. Who is that guy? That's a corpus luteum. The corpus luteum is only going to live for 12 days. Remember? Guess what? This little guy's smart. He's got his stuff together. And he's going to produce a substance called human, a hormone called human cor-choreonic gonadotropin, a.k.a. H.C.G. The embryo is going to produce H.C.G. And guess what that's going to do? Stay alive, man. Don't leave me now. So the corpus luteum is going to stay alive and keep producing progesterone because the embryo told it to. Now, are you ready for this? The human cor-choreonic gonadotropin is the hormone or is the substance that's detected on a pregnancy test. So when you pee out on your pregnancy stick, if there is H.C.G. in your urine, it means that there is an embryo and they're producing that in order to keep the corpus luteum alive. If the corpus luteum goes away, then you're going to start your period. And if the embryo is embedded in the wall of your uterus, you're going to start your period and then the embryo is going to come out with the period. So the whole setup is designed to make sure that it does not happen. Now, the uterus actually does make milk. It's called uterine milk, the food that the uterus feeds the embryo. But it's not called fallopian tube milk. I think that's a little unfair. And then, oh, this is cool too. Oh my gosh, it's so funny reading my lecture notes because they're full of exclamation points. Yeah, like, I just get so excited. It's very real. So the H.C.G. also causes baby boys to produce testosterone. But only if you're a boy baby, only if you have a Y chromosome will H.C.G. cause you to create testosterone. And the testosterone stimulates a small amount of things that happen during fetal development that make you into a boy if you have a Y chromosome. Like, oh my gosh, you have to go watch the anatomy lecture because we spent the whole, like a whole section talking about how we all start out with, you know, this little embryo starts out with these bipotential gonads which could be ovaries or could be testes. And H.C.G. is the hormone responsible for telling the bipotential gonad to become testes, not ovaries. And so then you get the gubernaculum. I mean, everybody's got to have a gubernaculum to pull the testes down out into the scrotum before the baby comes out. Thank you, H.C.G. for doing that work. Guess what else? Once the embryo is embedded in the uterus, the corpus luteum isn't going to live forever. The corpus luteum is only going to live for about three months. And so it is going to die eventually. And so the embryo's like, dude, I better come up with something quick. And also the lining of the uterus is very delicious, and it doesn't seem like it's going to last very long. So I better come up with another strategy. What's the other strategy? How is that little embryo going to stay alive? Well, it has to make a placenta. The placenta, I mean, I could draw you a picture of my little embryo with a little placenta attached to its belly button that's attached to the lining of the uterus. Oh, jeez. That's the mama. And I could draw her blood coming into this thing and then cruddy blood leaving. Yeah, okay, I could draw all that for you, but that's good. The placenta is where after three months, that's where food is exchanged and given to the baby. Oxygen and carbon dioxide are exchanged from the mom. The mom sends blood in, and the baby's blood comes in just tight contact, but no mixing. But that tight contact happens and then capillary exchange can take place across those capillaries. Oh, look, it says seven weeks. I said that the corpus luteum lived for three months. Cross that off. It's seven weeks. Aw, and then the placenta is going to take over progesterone production and make sure that the uterus stays happy and good. Do you love the reproductive system? Do you love every single system? Do you love every single topic in human physiology? Am I the luckiest person on the planet to get to do these things every single day? This is my job. I get paid for this? Okay. I won't tell you what hour it is. I don't know why I'm getting paid for this. I will see you later.