The development of tissues begins at the moment of fertilization, i.e., upon the union of a spermatozoon (Fig. A1) with an ovum (Fig. A2). The nucleus of the ovum and the head of the spermatozoon, both of which possess only half the full chromosomal complement, swell and form the female and male pronuclei. The two pronuclei double their deoxyribonucleic acid content, fuse, and thus form the nucleus of the first cell (zygote) of the new organism.