Concept 38.1   Fertilization Activates Development

• One sperm and only one sperm must fertilize an egg. Fertilization activates the egg.
• Egg and sperm make different contributions to the zygote. The egg contributes a haploid nucleus, most of the molecular machinery for cell division, energy production, and growth (yolk, mitochondria, ribosomes, etc.), as well as informational molecules such as transcription factors and mRNAs. The sperm contributes its nuclear material and usually its centrosome, the major microtubule-organizing center.
• Cytoplasmic contents of the egg often are not distributed uniformly. Nutrient molecules, usually in yolk granules, are often found in the vegetal hemisphere. In amphibians, rearrangement of cytoplasmic determinants at fertilization establishes the major axes of the future embryo. Review Figure 38.1 and Figure 38.2
• Fusion of egg and sperm nuclei produces a single-celled, diploid zygote.

Concept 38.2   Cleavage Creates Building Blocks and Produces a Blastula

• Cleavage is a period of rapid cell division without cell growth. Usually, little if any gene expression occurs during cleavage. Cleavage can be complete or incomplete. It results in a ball or mass of cells called a blastula. Review Figure 38.3 and Figure 38.4
• Cell membranes that form during cleavage segregate cytoplasmic determinants into different blastomeres.
• Some species undergo mosaic development, in which the fate of each cell is already determined at first division. Other species, including vertebrates, undergo regulative development, in which cell fates become determined later.
• Radial cleavage and spiral cleavage patterns are different. Both describe complete cleavage. Radial cleavage is the basal deuterostome pattern. Spiral cleavage occurs only in protostomes.
• The amount of yolk influences cleavage. Eggs with little yolk undergo complete cleavage. Very yolky eggs often undergo incomplete cleavage, so that part of the cytoplasm remains undivided.
• Early cell divisions in eutherian mammals are unique. These cell divisions produce a blastocyst composed of an inner cell mass that will become the embryo and an outer layer of cells called the trophoblast. The trophoblast helps the embryo implant into the uterine wall. Review Figure 38.8

Concept 38.3   Gastrulation Produces a Second, then a Third Germ Layer

• Gastrulation involves massive cell movements during which cells move from the outside to the inside of the embryo. It produces three germ layers and places cells from various regions of the blastula into new associations with one another. Review Figure 38.9
• The initial step of 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 Figure 38.9, Figure 38.10 and ANIMATED TUTORIAL 38.1
• Gastrulation in reptiles, including birds, differs from that in sea urchins and frogs because the blastula forms as a flattened disc of cells atop the yolk. Review Figure 38.11
• Although their eggs have very little yolk, placental mammals have a pattern of gastrulation similar to that of birds and other reptiles.

Concept 38.4   Gastrulation Sets the Stage for Organogenesis and Neurulation in Chordates

• Organogenesis is the process whereby tissues interact to form organs and organ systems.
• The dorsal lip of the amphibian blastopore is a critical site for cell determination and has been called the organizer. Review Figure 38.14, Figure 38.15 and ANIMATED TUTORIAL 38.2
• In the formation of the chordate nervous system, one group of cells that migrates through the blastopore becomes the notochord. The notochord induces the overlying ectoderm to form the neural tube. Review Figure 38.16
• Preventing the differentiation of ectoderm into epidermis allows ectoderm to become neural tissue.
• Neural crest cells are released from the closing neural tube. Neural crest cells migrate to different regions and form many parts of the vertebrate body—the sensory nervous system, bones of the head, pigment cells, and other structures. Review Figure 38.16
• Mesoderm on either side of notochord forms somites that will give rise to muscle, bone, dermis and other tissues, intermediate mesoderm that will form urinary and reproductive tissues, and lateral plate mesoderm that encloses the coelom and gives rise to the heart, muscles of the digestive system, and other tissues. Review Figure 38.16 and Figure 38.17

Concept 38.5   Extraembryonic Membranes Protect and Nourish the Growing Embryo

• Amniotic eggs contain four extraembryonic membranes. In reptiles, including birds, the yolk sac surrounds the yolk and provides nutrients to the embryo. The chorion lines the eggshell and participates with the allantois in gas exchange. The amnion surrounds the embryo and encloses it in an aqueous environment. The allantois is a storage sac for metabolic wastes and participates with the chorion in gas exchange (as part of the chorioallantoic membrane).
• Only the yolk sac and the allantois can develop blood vessels.
• The first blood cells and blood vessels appear in the yolk sac. Review Figure 38.18 and ACTIVITY 38.1
• In eutherian mammals, the chorion and allantois interact 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 38.19
• The yolk sac of fish is trilaminar, meaning it is composed of all three germ layers.

Concept 38.6   Development Continues throughout Life

• Development continues throughout life. To attain adult size, organisms undergo direct or indirect development.
• Not all parts of a body grow at the same rate. Unequal growth that results in differential growth of one body part relative to another is called allometry. Most direct developers display allometric growth.
• Morphologies of adult and larval forms of the same species may be very different. Larval forms serve two major functions—feeding and/or species distribution. Review Figure 38.22