51.1 Fertilization is the initial event in development.
Fertilization
• Fertilization consists of three stages: penetration, activation, and nuclei fusion. (p. 1082)
• The series of events initiated by sperm penetration are collectively called egg activation. (p. 1082)
• Sperm penetration stimulates the second meiotic division in the egg nucleus, may trigger movement of the egg cytoplasm around the point of sperm entry, and leads to an increase in metabolic activity. (p. 1082-1083)
51.2 Cell cleavage and the formation of a blastula set the stage for later development.
Cell Cleavage Patterns
• Cleavage is a period of cell division that is not accompanied by an increase in the overall volume of zygote cytoplasm and eventually leads to the formation of a hollow blastula. (p. 1084)
• The pattern of cleavage is influenced by the presence and location of yolk. (p. 1084)
51.3 Gastrulation forms the three germ layers of the embryo.
The Process of Gastrulation
• Gastrulation occurs when certain groups of cells invaginate and involute from the surface of the blastula, and determine the basic developmental pattern of the vertebrate embryo. (p. 1088)
• By the end of gastrulation, embryo cells have rearranged themselves into three primary germ layers: ectoderm, mesoderm, and endoderm. (p. 1087)
51.4 Body architecture is determined during the next stages of embryonic development.
Developmental Processes During Neurulation
• Development of the dorsal nerve cord is termed neurulation. (p. 1090)
• The appearance of the neural crest was a key event in vertebrate evolution, ultimately developing into structures characteristic of the vertebrate body. (p. 1090)
• In primitive chordates, pharyngeal slits evolved into the vertebrate gill chamber, improving respiration. (p. 1091)
How Cells Communicate During Development
• The relative position of particular cell layers helps determine the organs that develop. (p. 1092-1093)
• Inducer cells produce a protein factor that binds to the cells of the target tissue, initiating changes in gene expression. (p. 1093)
• Development is a process of progressive restriction of gene expression. (p. 1093)
• At the stage called commitment, every cell's fate becomes fixed. (p. 1093)
Embryonic Development and Vertebrate Evolution
• Patterns of chordate development have built up in incremental steps over the evolutionary history of those groups. (p. 1094-1095)
• Biogenic law states that embryological development (ontogeny) exhibits a progression of changes indicative of evolutionary origin (phylogeny). (p. 1095)
• The embryonic stages of a particular vertebrate often reflect the embryonic stages of its ancestors. (p. 1095)
Extraembryonic Membranes
• Extraembryonic membranes become the fetal membranes -- amnion, chorion, yolk sac, and allantois -- and serve to support embryonic development. (p. 1096)
51.5 Human development is divided into trimesters.
First Trimester
• The embryo reaches the uterus six to seven days after fertilization. Gastrulation takes place in the second week, while neurulation occurs in the third week, and organogenesis begins during the fourth week. (pp. 1097-1098)
• Morphogenesis takes place in the second month as the embryo assumes its adult shape. (p. 1100)
• The eighth week marks the transition from an embryo to a fetus. (p. 1100)
• In the third month, the nervous system develops, and the arms and legs start to move. (p. 1100)
Second and Third Trimesters
• During the fourth month, bones actively enlarge, and the mother can feel the baby kicking. (p. 1100)
• Growth begins in earnest in the sixth month. (p. 1100)
• The third trimester is predominantly a period of growth rather than development. (p. 1100)
• During this period, most of the major nerve tracts in the brain are formed. (p. 1100)
Birth and Postnatal Development
• Oxytocin and prostaglandins help stimulate uterine contractions, assisting the movement of the fetus downward. (p. 1101)
• During pregnancy, the mammary glands are prepared for, but prevented from, lactating. (p. 1101)
• Oxytocin stimulates contraction of the smooth muscle around the alveolar ducts, causing milk to be ejected. (p. 1101-1102)
• Babies typically double their birth weight within two months. (p. 1102)
• In most mammals, brain growth is predominantly a fetal phenomenon, while in human infants, the brain continues to grow for the first few years of postnatal life. (p. 1102)