...And more ⚽⚽!

Eliza!

Ok, so yesterday I was thinking a bunch about the evolution of scrotums to answer the question — why don't elephants have them!? But then I onviously just got into scrotum anatomy and scrotum embryological development more generally. I wanted to talk to you about it because I find it so interesting!

My understanding is currently superficial but as of right now, this is my sense of things:

Testicles and ovaries are from the same tissue which is located in the middle region of an early-stage embryo, near the future kidneys. Some cells from the outer layer of the embryo use chemical signals to migrate into the area that will develop into the testes and ovaries, and these cells will eventually become the germ cells (as in germination cells) which will become sperm and eggs in that person.

In fetuses, there's this ligament (called the gubernaculum) that helps to hold the testes/ovaries in place.

In fetuses that become "girls," one end of this ligament is attached to the future ovary and the other end attaches to the uterus and then folds down to attach to the labia majora.

In fetuses that become "boys," a gene turns on which causes the shortening of this ligament which pulls the testes into what will become the scrotum. On this journey, the testes go through a little tunnel in abdominal wall called the inguinal canal, which both males and females have, but which is bigger in men, with more stuff going through it. A fetuses which will become a "boy" has testes which descen through this dunnel into the future scrotum as the shortening gubernuculum pulls it through. And for reasons, I don't fully understand yet (i.e. perhaps because of the testis' act of travelling through this tunnel in a back-to front and down action, or because of the size of the stuff running through it) this tunnel is an especially weak spot in men's abdominal cavities. Thus, the descent of the testes into the scrotum is the reason why men are more prone to getting hernias in this region where their abdominal wall tear and parts of their intestines bulge making their muscle and skin protrude .

Another thing I thought was cool while reading about testes is that one reason why, when "boys" get hit in the balls, they feel pain in their abdomen, is because, along with ligamendt and spermatic chords that go from the testes through the inguinal canal, there are also neves that connect the testes to the abdomen. That's why when they get hit in the ⚽⚽ with a ⚽, they fold over and grab their stomach! Makes sense, but I didn't realize that the connection was so literal! I think elephants have a good idea in having their balls on the inside;)

Some resources:

• This great animation about formation of inguinal canal in fetuses
• This video which shows dissected tissue and a diagram about how the testes descend.
• This video describing the anatomy of the inguinal canal

P.S. Here's another letter!