What did spermists believe?

Answer 1

Spermism or "encasement" was an early sexual reproductive theory related to the proposed existence of homunculi and animalcules, tiny versions of adult organisms within the sex cells.

The concept was that the human sperm cell was the literal forebear of the eventual fetus, having been "encased" within the egg cell. The head of the sperm cell became the head and the tail became the body, as in tadpoles becoming frogs. However, a multicellular embryo (zygote) does not retain any of the physical aspects of the haploid sperm cell.