Dad, I heard my friends talking about something called sexual reproduction. They said that internal fertilization is how you make babies. What is it?
Well son, sexual reproduction is a complex process of growth and development. It starts with a dioecious organism's gonads. The gonads are where the gametes are produced. The female has eggs that it creates through oogenesis, and the male has sperm that it creates through spermatogenesis.
Ok, I understand how gametes are created, but what what does that have to do with what my friends are talking about?
Ah, that. Well, the male and the female come together, and they, uh, mate. When they mate, sperm is deposited in the female, and eventually it meets the female's egg. When these two cells meet, the acrosome in the tip of the sperm cell allows it to burrow into the egg cell. Once this happens, the egg has a cortical reaction to stop other sperm from trying to fertilize it.
What happens after that?
After the sperm meets the egg and they fuse, a zygote is formed. The zygote undergoes cleavage many times, which splits it from the original two gametes into a blastomere. The blastomere starts with the morula after several divisions, and after that the blastula is formed. Once the cells divide to 16-32 cells, the blastocoel forms, creating a cavity.
Then what happens dad?
Then. gastrulation begins. This is the process of the blastula turning into the gastrula. When this happens, germ layers start to form, differentiating the cells. The first germ layer that forms is the ectoderm, which will form the nervous system and epidermis.
What about the other germ layers?
Well, the endoderm forms after the ectoderm, creating the muscle cells and connective tissue in the body. After that, the mesoderm forms, creating columnar cells in the digestive system and many internal organs.
And then what happens?
After the germ layers form, the blastopore and coelom form, which is the cavity in the gastrula. Eventually, the cavity will go all the way through. The gastrula forms differently depending on what kind it is. If the gastrula is a deuterostome, the cavity will form the anus. In a protostome, it will form the mouth. After this, the organs form in a process called organogenesis.