do all animals develop from embryos?

Answer 1

Not all of them no.

The vast bulk of them do but, for example the conception/growth/birth of insects is different.

There are also many aqatic species that do not have an embryo stage as well.

People tend to think of animal as... birds, fish, mammals, etc and forget that "animals" also includes species like sea anemonies and various coral.

But the short answer... nope.

Answer 2

Multicellular animals start off as a single fertilized egg cell. The cell divides into two, each of them into two, etc. In some species the original division already starts off embryogenesis (the formation of an organism with cells that are not all identical) but in vertebrates all the cells are fully pluripotent for at least a few generations (that's how identical twins happen, the identical cells separate and each goes on to form its own separate organism). At some point, however, individual cells are programmed into a separate developmental fate, going on the develop into specific types of tissues. Some of those cells become part of the new organism and some are "scaffolding", needed for embryonic life but discarded when the organism starts to live separately (think of the placenta, which is also derived from the fertilized ovum but is discarded at birth).

There are some communal organisms which are actually single-cell organisms that get together to form an animal-like body for part of the life cycle. Slime molds, for example, are communities of amoebae that get together for transport and resource exploitation. Slime molds are not, strictly speaking, animals, and do not go through embryogenesis.