Sperm travel from the testis through the epididymis, vas deferens, ejaculatory duct, and urethra. The urethra carries semen through the penis to the exterior.

Oogonia and primary oocytes are diploid. After meiosis I, the secondary oocyte and first polar body are haploid. Completion of meiosis II produces a haploid ovum and second polar body.

After implantation, hCG maintains the corpus luteum, which continues secreting progesterone and estrogen to support the endometrium. These hormones suppress pituitary LH and FSH. Basal body temperature commonly remains elevated. Later, the placenta takes over much of the hormone production.

I, III, and IV are true. Monozygotic twins originate from one zygote and share nearly the same inherited genome, but their physical traits and fingerprints need not be identical. Dizygotic twins result from two separate fertilizations. The male urethra carries urine and semen at different times.

Endoderm forms most of the epithelial lining of the respiratory and digestive tracts and the epithelial components of associated glands. Supporting connective tissue and much of the smooth muscle are derived from mesoderm. Somites and the notochord are also mesodermal.

The mammalian zona pellucida is an extracellular glycoprotein layer surrounding the oocyte, not an intracellular matrix. It participates in sperm recognition and changes after fertilization to help prevent polyspermy.

All four statements are true, so none is false. Vaginal acidity, maintained largely by lactic acid associated with the microbiota, limits many microbes. Unequal cytokinesis during oogenesis retains most cytoplasm in one functional gamete. The LH surge triggers ovulation, and the ovarian cycle includes follicular and luteal phases.

The epidermis develops from ectoderm. Most muscle, skeletal, circulatory, and lymphatic tissues develop from mesoderm.

The single umbilical vein carries oxygen- and nutrient-rich blood from the placenta toward the fetus. The two umbilical arteries carry fetal blood back to the placenta. Maternal blood surrounds the chorionic villi, while fetal blood remains in villous capillaries. Substances cross the placental barrier; the two circulations are not directly connected.

Allometric growth changes body proportions because body parts grow at different relative rates. Cell differentiation produces specialized cell types. Induction occurs when signals from one group of cells influence another group’s development. Morphogenesis produces an organism’s shape and tissue organization. Cytoplasmic determinants are molecules in the egg cytoplasm that influence cell fate.

The acrosome is a vesicle over the sperm head that contains hydrolytic enzymes. During the acrosomal reaction, these enzymes help the sperm pass through the egg’s protective coverings. Fusion of the sperm and oocyte plasma membranes then permits sperm entry.

Fertilization usually occurs in the ampulla of the uterine (fallopian) tube, also called the oviduct. The developing embryo then travels toward the uterus.

The corpus luteum secretes progesterone and estrogen. LH and FSH are secreted by the anterior pituitary. A follicle contains an oocyte surrounded by somatic follicular cells, and a blastocyst normally implants in the uterine endometrium.

Cortical granules release their contents by exocytosis. These contents modify and elevate the vitelline layer to form the fertilization envelope, producing the slow block to polyspermy. Membrane depolarization produces the fast block; extension of the acrosomal process is part of the acrosomal reaction.

Fusion of sperm and egg plasma membranes triggers a rapid change in egg membrane potential. This depolarization provides a temporary fast block to additional sperm entry. The cortical reaction subsequently produces the slower, longer-lasting block.

Early cleavage divisions produce a morula and then a blastula (a blastocyst in mammals). Gastrulation establishes the germ layers. Neurulation then forms the neural tube. Inductive signaling operates during multiple developmental processes and is not a single final stage.

Maternal arteries deliver blood to spaces surrounding the chorionic villi. Oxygen and nutrients cross into fetal capillaries, and the umbilical vein carries fetal blood toward the fetus. The umbilical arteries return fetal blood to the placenta. Maternal blood drains through maternal veins. The arrows summarize transfer directions, not direct connections between maternal and fetal vessels.

All four statements are true. Hemoglobin synthesis begins before birth, and adult hemoglobin gradually becomes predominant during infancy. Lung expansion reduces pulmonary vascular resistance and increases pulmonary blood flow. Left atrial pressure rises above right atrial pressure, normally causing functional closure of the foramen ovale. Anatomical sealing occurs later and is not complete in every person.

I, III, and IV are true. During oogenesis, unequal cytokinesis retains most of the cytoplasm in the oocyte rather than dividing it equally among the ovum and polar bodies. Meiosis I reduces the chromosome set from diploid to haploid, producing a secondary oocyte and first polar body. The ruptured follicle develops into the corpus luteum.

After ovulation, the corpus luteum develops and secretes increasing progesterone, along with estrogen. Progesterone supports the secretory endometrium and provides negative feedback that helps prevent another ovulatory surge. Estradiol has a major preovulatory peak and a smaller luteal rise.

Human fertilization triggers completion of meiosis II in the secondary oocyte. Subsequent cleavage divisions of the zygote are mitotic, not meiotic. Cortical granule release modifies the zona pellucida to help prevent polyspermy. Fertilization usually occurs in the uterine tube, and the blastocyst normally implants in the uterus.

A blastula surrounds a fluid-filled cavity called the blastocoel, whereas a morula is a compact ball of cells. Gastrulation establishes the germ layers; triploblastic animals form ectoderm, mesoderm, and endoderm. The blastopore is an opening into the developing primitive gut, or archenteron. Organs develop during organogenesis.

The yolk sac surrounds the yolk and transfers nutrients to the embryo. The allantois stores nitrogenous wastes and works with the chorion in gas exchange. The amnion encloses the embryo in fluid that cushions it and helps prevent dehydration.

The epididymis is the main site of sperm maturation and storage. The vas deferens primarily transports sperm toward the ejaculatory duct; it is not the primary site of sperm maturation. Leydig cells produce testosterone, and the prostate adds enzyme-containing fluid to semen.

Estrogen and a progestin provide negative feedback that suppresses FSH and LH, including the ovulatory LH surge. Progestins also thicken cervical mucus. These effects explain the hormonal contraceptive mechanism.

The midcycle LH surge and smaller FSH surge occur shortly before ovulation. The major estradiol peak precedes this surge. Progesterone peaks during the luteal phase, and estradiol has a smaller luteal rise.

Tooth enamel develops from oral ectoderm. Heart muscle and skeletal muscle are mesodermal. Respiratory and pancreatic epithelia are derived from endoderm. Whole organs can contain supporting tissues derived from other germ layers.

Gastrulation establishes the germ layers and produces the archenteron and blastopore. In amphibians, gastrulation movements begin at the dorsal lip of the blastopore. During neurulation, the neural plate folds to form the neural tube.

Amphibians are anamniotes and lack an amnion. In amniotes, including humans, the amnion surrounds the embryo in a fluid-filled sac that cushions it.

The placenta is the exchange organ between mother and fetus. Oxygen and nutrients cross the placental barrier toward fetal blood, while carbon dioxide and other wastes move toward maternal blood. Maternal and fetal blood normally remain separate.

All four statements are true, so none is false. FSH acts through Sertoli cells, while LH stimulates Leydig cells to produce testosterone. Spermatogenic meiosis produces four haploid spermatids through relatively symmetric cytokinesis; excess cytoplasm is lost later during sperm maturation. Flagellar movement requires ATP.

In shelled amniotic eggs, the allantois stores nitrogenous wastes and participates with the chorion in gas exchange. In humans, the allantoic stalk contributes to the umbilical cord structure. The chorion contributes the fetal portion of the placenta, while maternal endometrial tissue forms the maternal portion; the allantois does not become the whole placenta.