The "undeveloped egg" and the material you have pictured cut open is Lash Egg or Salpingitis.
This is inflammation of the oviduct.
If caught early, you can try treating with an antibiotic to help with infection and hopefully extend life.
Tetracyclines, Amoxicillin or Enrofloxacin are commonly used to treat Salpingitis.
Here's some reading that will explain more:
https://the-chicken-chick.com/salpingitis-lash-eggs-in-backyard/
https://the-chicken-chick.com/causes-of-lash-eggs-salpingitis-by/
This is inflammation of the oviduct.
If caught early, you can try treating with an antibiotic to help with infection and hopefully extend life.
Tetracyclines, Amoxicillin or Enrofloxacin are commonly used to treat Salpingitis.
Here's some reading that will explain more:
https://the-chicken-chick.com/salpingitis-lash-eggs-in-backyard/
https://the-chicken-chick.com/causes-of-lash-eggs-salpingitis-by/
