 Now, I know I definitely have many times where I feel like, dude, are male humans like another species entirely? Like, where do these things come from? And I'm sure that the feeling is mutual. But the fact is that up until, like, if you looked at a little baby, like we just made a baby, that was awesome. It happened in the fallopian tubes. That little zygote is now down in the uterus. It is growing every day, going through mitosis, becomes an embryo. When that little guy is six weeks old, if you looked in there, there's nothing that would tell you whether it was male or female. And in fact, if you could treat it with boy hormones, you can make it a boy. If you treat it with not boy hormones, you can treat it with a girl, which is kind of an interesting thing to start thinking about. Let's try to avoid that. They all look like this. They have a structure called a bipotential gonad that will, under the influence of boyness, what is boyness? What does that even mean? What's the actual difference between males and females? The source of all differences between males and females. What's the chromosome? The fellas have one, the ladies lack one. We have two Xs, the fellas have an X and a Y. The presence of the Y chromosome gives you male characteristics. The Y chromosome acts on the bipotential gonad and causes it to descend with the help of the gubernaculum into the scrotum. The Y chromosome, take a look at this. This bipotential gonad is going to be just, well, if nothing had the Y chromosome isn't there, it's going to become an ovary. You see these two sets of tubes that are traveling down and they're kind of associated with the bipotential gonad. If there's a Y chromosome, one of the sets of tubes kind of disintegrates. And the other set of tubes becomes the vas deferens, the epididymis, all the structures that are required for maleness. If there is no Y chromosome, then the other set of tubes disintegrates and the remaining tube becomes the fallopian tubes and part of the uterus. Seriously? How awesome is that? We actually were identical all the way to six weeks in utero. Now what that means is that this bipotential gonad that we speak of is actually their homologous structures. And so let's take a second to think about the homologies. So if you have an X and a Y, your gonad is going to be what? The testes. Your bipotential gonad is going to become testes. If you are a X and an X, then your bipotential gonad is going to become an ovary. Your bipotential gonads are going to become ovaries. Are you good? If you have an X and a Y, your testes will produce sperm. If you have two Xs, your ovaries will produce eggs. Should we just say oocytes of some flavor? Because eggs we only produce if we fertilize. That works for me. Now this is the most interesting part. If you have a Y chromosome, then you're going to produce a structure. Your testes are going to be pulled out of your body cavity through your inguinal canal out into this little sac. In fact, I'm going to start with the ladies. The ladies leave their bipotential gonads in their peritoneal cavities and they have labia majora. Aw, I just made it blue. I can't have that. They have labia majora in green. And remember, labia majora was the big outer lips that have hair on them after puberty. Labia majora, two flaps. Guess what? If you have a Y chromosome, your labia majora zips up. What? It does. It zips up closed and forms the scrotum. These are homologous structures. Oh, guess what? It gets cooler. Labia minora has to become something. Remember labia minora? Labia minora is the inner little flaps lateral to the vagina and they do not have hair on them because their homologous pair is the ventral side of the penis. So labia majora zips up and becomes the scrotum. Labia minora zips up and becomes the ventral side of the penis. Now, the ventral side of the penis, you have to point the penis straight up and then that surface that's facing you, that's the ventral side of the penis. And you can see there's just like a little zipline. One more. Oh. Oh, this one is super important. This one's like the most important. What about the penis? No. What about the glands of the penis? This is, I think, the glands of the penis. Who would you guess it's homologous to? The clitoris. All right. You're not so different from those with differing chromosomes. It's dense lecture, but let me just tell you this. It's dense because a lot of this conversation would be happening in lab because a lot of this conversation is dealing with lab structures that we'd be looking at. So all I've done is I've taken all of that and given it to you, which means you've, essentially, if you've completed this lecture, you've totally prepped for lab, right? No, you prepped for lab. Okay, I'm out of here. Yay, bye-bye now.