The placental barrier between the maternal and foetal blood is minimum in

The mother germ cells are transformed into a mature follicle through series of steps. Provide the missing steps in the blank boxes.

$\boxed{{\text{Oogonia}}}$ $\to $ $\boxed{\text{A}}$ $\to $ $\boxed{{\text{Primary follicle}}}$ $\to $ $\boxed{\text{B}}$ $\to $$\boxed{\text{C}}$ $\to $ $\boxed{{\text{Mature or Graafian follicle}}}$

There is a connective tissue cord extending between the testis and abdominal wall called

The second maturation division of the mammalian ovum occurs

Pasteels $(1945)$ described gastrulation takes place in aves by