Embryology
Embryology, branch of biology dealing with the development of the animal embryo. (For the embryology of plants, see Fertilization ; Plant ; Seed .) Embryology includes within its province the development of the fertilized egg and embryo and the growth of the fetus . Comparative Embryology Sea urchins, frogs, humans, and many other animals are remarkably similar in their early development. All begin with a single cell that divides into two cells, the first step in the process of cleavage (1a, 2a, 3a). During cleavage, cell divisions occur so rapidly that the cells do not have time to grow between divisions, and the result is smaller and smaller cells. Cleavage produces a solid ball of cells called a morula (1b, 2b, 3b). Within the morula, a fluid-filled cavity called the blastocoel develops, converting a morula into a blastula (1c, 2c, 3c). In a process called gastrulation, certain cells of the blastula migrate to different regions of the blastula to create the gastrula, a structure wit...