Fertilization (pp. 735–737)

1. Upon fertilization of a secondary oocyte by a spermatozoon in the uterine tube, meiotic development is completed and a diploid zygote is formed.

2. Morphogenesis is the sequential formation of body structures during the prenatal period of human life. The prenatal period lasts 38 weeks and is divided into a preembryonic, an embryonic, and a fetal period.

3. A capacitated spermatozoon digests its way through the zona pellucida and corona radiata of the secondary oocyte to complete the fertilization process and formation of a zygote.

Preembryonic Period (pp. 737–742)

1. Cleavage of the zygote is initiated within 30 hours and continues until a morula forms; the morula enters the uterine cavity on about the third day.

2. A hollow, fluid-filled space forms within the morula, at which point it is called a blastocyst.

3. Implantation begins between the fifth and seventh day and is enabled by the secretion of enzymes that digest a portion of the endometrium.
   1. During implantation, the trophoblast cells secrete human chorionic gonadotropin (hCG), which prevents the breakdown of the endometrium and menstruation.

   2. The secretion of hCG declines by the tenth week as the developed placenta secretes steroids that maintain the endometrium.

4. The embryoblast of the implanted blastocyst flattens into the embryonic disc, from which the primary germ layers of the embryo develop.
   1. Ectoderm gives rise to the nervous system, the epidermis of the skin and epidermal derivatives, and to portions of the sensory organs.

   2. Mesoderm gives rise to bones, muscles, blood, reproductive organs, the dermis of the skin, and connective tissue.

   3. Endoderm gives rise to linings of the GI tract, digestive organs, the respiratory tract and lungs, and the urinary bladder and urethra.

Embryonic Period (pp. 742–752)

1. The events of the 6-week embryonic period include the differentiation of the germ layers into specific body organs and the formation of the placenta, the umbilical cord, and the extraembryonic membranes. These events make it possible for morphogenesis to continue.

2. The extraembryonic membranes include the amnion, yolk sac, allantois, and chorion.
   1. The amnion is a thin membrane surrounding the embryo. It contains amniotic fluid that cushions and protects the embryo.

   2. The yolk sac produces blood for the embryo.

   3. The allantois also produces blood for the embryo and gives rise to the umbilical arteries and vein.

   4. The chorion participates in the formation of the placenta.

3. The placenta, formed from both maternal and embryonic tissue, has a transport role in providing for the metabolic needs of the fetus and in removing its wastes.
   1. The placenta produces steroid and polypeptide hormones.

   2. Nicotine, drugs, alcohol, and viruses can cross the placenta to the fetus.

4. The umbilical cord, containing two umbilical arteries and one umbilical vein, is formed as the amnion envelops the tissues on the underside of the embryo.

5. From the third to the eighth week, the structure of all the body organs, except the genitalia, becomes apparent.
   1. During the third week, the primitive node forms from the primitive line, which later gives rise to the notochord and intraembryonic mesoderm.

   2. By the end of the fourth week, the heart is beating; the primordial tissues of the eyes, brain, spinal cord, lungs, and digestive organs are properly positioned; and the superior and inferior limb buds are recognizable.

   3. At the end of the fifth week, the sense organs are formed in the enlarged head and the appendages have developed with digital primordia evident.

   4. During the seventh and eighth weeks, the body organs are formed, except for the genitalia, and the embryo appears distinctly human.

Fetal Period (pp. 752–755)

1. A small amount of tissue differentiation and organ development occurs during the fetal period, but for the most part fetal development is primarily limited to body growth.

2. Between weeks 9 and 12, ossification centers appear, the genitalia are formed, and the digestive, urinary, respiratory, and muscle systems show functional activity.

3. Between weeks 13 and 16, facial features are formed and the fetal heartbeat can be detected with a stethoscope.

4. During the 17-to-20-week period, quickening can be felt by the mother, and vernix caseosa and lanugo cover the skin of the fetus.

5. During the 21-to-25-week period, substantial weight gain occurs and the fetal skin becomes wrinkled and pinkish.

6. Toward the end of the 26-to-29-week period, the eyes have opened, the gonads have descended in a male, and the fetus is developed to the extent that it might survive if born prematurely.

7. At 38 weeks, the fetus is full-term; the normal gestation is 266 days.

Labor and Parturition (pp. 755–756)

1. Labor and parturition are the culmination of gestation and require the action of oxytocin, secreted by the posterior pituitary, and prostaglandins, produced in the uterus.

2. Labor is divided into dilation, expulsion, and placental stages.

Periods of Postnatal Growth (pp. 756–762)

1. The course of human life after birth is seen in terms of physical and physiological changes and the attainment of maturity in the neonatal period, infancy, childhood, adolescence, and adulthood.

2. The neonatal period, extending from birth to the end of the fourth week, is characterized by major physiological changes.
   1. The most critical need of the newborn is to establish adequate respiratory and heart rates. A normal respiratory rate is 30 to 40 respirations per minute, and a normal heart rate ranges from 120 to 160 beats per minute.

   2. The four reflexes in the newborn critical to survival are the suckling reflex, the rooting reflex, the crying reflex, and the breathing reflex.

3. Infancy, extending from 4 weeks through the second year, is characterized by tremendous growth, increased coordination, and mental development.
   1. By 2 years, most infants weigh about four times their birth weight and average between 32 and 36 inches in length.

   2. Growth is a differential process resulting in gradual changes from infant to adult body proportions.

4. Childhood, extending from the end of infancy to adolescence, is characterized by steady growth until preadolescence, at which time there is a marked growth spurt.
   1. During childhood, the average child becomes thinner and stronger each year as he or she grows taller.

   2. The fact that disease and death are relatively rare during childhood may be due to the fact that lymphoid tissue is at its peak of development at this time; it is also present in greater amounts in children than in adults.

5. Adolescence is the period of growth and development between childhood and adulthood.
   1. Puberty is the stage of early adolescence when the secondary sex characteristics are expressed and the sex organs become functional.

   2. The end result of puberty is the structural expression of gender, or sexual dimorphism.

   3. Menarche generally occurs in adolescent girls at the age of 13, but it may range from 9 to 17 years. At this time, vaginal secretions change from alkaline to acid.

   4. The first physical indications of puberty are the appearance of breast buds in females and the growth of the testes and the appearance of sparse pubic hair in males.

   5. Although semen may be ejaculated at age 13, sufficient mature spermatozoa for fertility are not produced until 14 to 16 years of age.

6. Adulthood, the final period of human physical change, is characterized by gradual senescence as a person ages.
   1. Although skeletal maturity is reached in early adulthood, anatomical and physiological changes continue throughout adulthood and are part of the aging process.

   2. Sexual dimorphism in human adults is evident anatomically, physiologically, metabolically, and behaviorally.

   3. Male and female differences in the stature, proportions, and composition of the body may become more apparent with age.

Inheritance (pp. 762–765)

1. Inheritance is the passage of hereditary traits carried on the genes of chromosomes from one generation to another.

2. Each zygote contains 22 pairs of autosomal chromosomes and 1 pair of sex chromosomes—XX in a female and XY in a male.

3. A gene is the portion of a DNA molecule that contains information for the production of one kind of protein molecule. Alleles are different forms of genes that occupy corresponding positions on homologous chromosomes.

4. The combination of genes in an individual’s cells constitutes his or her genotype; the observable expression of the genotype is the person’s phenotype.
   1. Dominant alleles are symbolized by uppercase letters and recessive alleles are symbolized by lowercase letters.

   2. The three possible genotypes are homozygous dominant, heterozygous, and homozygous recessive.

5. A Punnett square is a convenient means for expressing probability.
   1. The probability of a particular genotype is 1 in 4 (.25) for homozygous dominant and homozygous recessive, and 1 in 2 (.50) for heterozygous.

   2. A single trait is studied in a monohybrid cross; two traits are studied in a dihybrid cross.

6. Sex-linked traits such as color blindness and hemophilia are carried on the sex-determining chromosome.