Summary
This section covers the continuum of animal development, beginning with the fusion of gametes into a zygote and progressing through cleavage, blastula and gastrula formation, and the establishment of germ layers through embryonic induction. It highlights key mechanisms including cytoplasmic localization, differential gene expression, apoptosis, and cell migration, all crucial for shaping complex body plans. In human development, the text distinguishes between the embryonic and fetal periods, describes the formation and function of extraembryonic membranes and the placenta, and delineates the processes of labor, birth, and lactation. Understanding these processes is essential for grasping both normal development and the origin of developmental disorders.