Which part of the sperm is known as the powerhouse?

During the first meiotic division,into what do the primary spermatocytes differentiate?

After ovulation,the mammalian ovum is surrounded by which of the following layers?

The provided image is completely unrelated to the question text. The question asks for a diagrammatic presentation of developing follicle events in the human ovary. Based on the standard biological sequence of ovarian follicle development:
$1$. Primary follicles develop into secondary follicles.
$2$. Secondary follicles develop into tertiary follicles.
$3$. Tertiary follicles develop into the Graafian (mature) follicle.
$4$. After ovulation,the ruptured follicle transforms into the corpus luteum.
Given the standard sequence,identify the correct labels for $C$ and $E$ in a typical diagram of this process.

When do secondary sexual characteristics first appear?

Meiotic division during oogenesis is different from that in spermatogenesis. Explain how and why?