Of the four chambers of the human heart, which has the thickest walls, and why?

Why can you not catch the same cold twice, but a new cold will make you sick for a week or two?

1. What is the key step in spermatogenesis that allows for unlimited sperm production? This step is not present in oogenesis, and therefore only a finite number of eggs could be produced.
2. What is the key step in spermatogenesis that allows for unlimited sperm production? This step is not present in oogenesis, and therefore only a finite number of eggs could be produced.