Immediately after the cells differentiate—become particular kinds of cells—they begin to form tissues. when does heart tissue first form in a human?

19-22 days after fertilization of egg by sperm

Explanation:

The heart is the first organ to be formed in a developing embryo. This is because it is a vital organ and responsible for the circulation of blood (with oxygen and nutrients) to the other developing tissues. It begins to be formed at around 18 days from fertilization and begins to function after day 21.  

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.