Concept 33.1 Fertilization Activates Development

• The sperm and egg make different contributions to the zygote. The egg contributes a haploid nucleus, nutrients, ribosomes, mitochondria, mRNAs, and proteins. The sperm contributes a haploid nucleus and, in most species, a centriole.
   
• In amphibians, the cytoplasmic contents of the egg are not distributed homogeneously, and they are rearranged after fertilization to set up the major axes of the future embryo. The nutrient molecules are generally found in the vegetal hemisphere, whereas the nucleus is found in the animal hemisphere. Review Figures 33.1 and 33.2
   

Concept 33.2 Cleavage Repackages the Cytoplasm of the Zygote

• Cleavage is a period of rapid cell division without cell growth. Except in mammals, little if any gene expression occurs during cleavage. Cleavage can be complete or incomplete. The result of cleavage is a ball or mass of cells called a blastula. Review Figure 33.3
   
• Early cell divisions in mammals are unique in being slow and allowing for gene expression. These cell divisions produce a blastocyst composed of an inner cell mass that becomes the embryo and an outer layer of cells that becomes the trophoblast. The trophoblast helps the blastocyst implant in the uterine wall. Review Figures 33.4 and 33.5
   
• Some species undergo mosaic development, in which the fate of each cell is determined during early divisions. Other species, including vertebrates, undergo regulative development, in which remaining cells can compensate for cells lost in early cleavages.
   

Concept 33.3 Gastrulation Creates Three Tissue Layers

• Gastrulation involves massive cell movements that produce three germ layers and place cells from various regions of the blastula into new associations with one another. Review Figure 33.6
   
• The initial step of sea urchin and amphibian gastrulation is inward movement of certain blastomeres. The site of inward movement becomes the blastopore. Cells that move into the blastula become the endoderm and mesoderm; cells remaining on the outside become the ectoderm. Review Figures 33.6 and 33.7 and ANIMATED TUTORIAL 33.1
   
• The dorsal lip of the amphibian blastopore is a critical site for cell determination. It has been called the organizer because it induces determination in cells that pass over it during gastrulation. Review Figures 33.7, 33.8, 33.9 and ANIMATED TUTORIAL 33.2
   
• The protein β-catenin activates a signaling cascade that induces the primary embryonic organizer and sets up the anterior–posterior body axis.
   
• Gastrulation in reptiles and birds differs from that in sea urchins and frogs because the large amount of yolk in reptile and bird eggs causes the blastula to form a flattened disc of cells. Review Figure 33.11
   
• Although their eggs have no yolk, placental mammals have a pattern of gastrulation similar to that of reptiles and birds.
   
• Gastrulation is followed by organogenesis, the process whereby tissues interact to form organs and organ systems.
   

Concept 33.4 Neurulation Creates the Nervous System

• In the formation of the vertebrate nervous system, one group of cells that migrates over the blastopore lip becomes the notochord. The notochord organizes the overlying ectoderm to form a neural tube. Review Figure 33.12
   
• The anterior region of the neural tube becomes the brain. Different regions differentiate into the forebrain, midbrain, and hindbrain. The posterior region of the neural tube becomes the spinal cord. Review Figure 33.13
   
• The notochord and neural crest cells participate in the segmen-tal organization of mesoderm into structures called somites along the body axis. Rudimentary organs and organ systems form during these stages. Review Figure 33.14
   

Concept 33.5 Extraembryonic Membranes Nourish the Growing Embryo

• Amniote eggs contain four extraembryonic membranes. In a bird or reptile, the yolk sac surrounds the yolk and provides nutrients to the embryo, the chorion lines the eggshell and participates in gas exchange, the amnion surrounds the embryo and encloses it in an aqueous environment, and the allantoic membrane forms the allantois, a storage sac for metabolic wastes. Review Figure 33.15 and WEB ACTIVITY 33.1
   
• In placental mammals, the chorion interacts with maternal uterine tissues to form a placenta, which provides the embryo with nutrients and gas exchange. The amnion encloses the embryo in an aqueous environment. Review Figure 33.16
   
• Development continues throughout life.