Fertilization occurs when sperm and egg nuclei unite to form an embryo. Primitive fertilization involves genetic material exchange between cells. Membrane fusion creates channels for cell-to-cell material transfer
System found in mammals, insects, snakes, fish, and some plants. Females have XX chromosomes, males have XY chromosomes. Y chromosome presence triggers male development in humans. Most species require at least one X chromosome for survival
Somites are paired blocks of paraxial mesoderm forming along head-to-tail axis. Mesoderm forms simultaneously with ectoderm and endoderm. Somites appear simultaneously on both sides of neural tube. Development occurs every 90 minutes in chick embryos, 2 hours in mice
Motor neurons originate in motor cortex, brainstem or spinal cord. Upper motor neurons originate in cerebral cortex and travel to brain stem/spinal cord. Lower motor neurons originate in spinal cord and control muscles and glands
Ectoderm is the outermost germ layer in early embryonic development. Derives from outer germ cells and differentiates into epithelial and neural tissues. Divided into dorsal surface ectoderm and neural plate in vertebrates. Forms from animal hemisphere during gastrulation process
Baby's sex is determined at fertilization by sperm and egg chromosomes. Eggs always have X chromosome, sperm can have X or Y. Only sperm determines baby's sex, not either parent. Up to 1 billion sperm released during ejaculation