Sperm travel from the testis through the epididymis, vas deferens, ejaculatory duct, and urethra to the external urethral opening. The epididymis is the site of sperm maturation and principal storage.

The oogonium and primary oocyte are diploid cells. The secondary oocyte, first polar body, ovum, and second polar body are haploid cells.

A blastocyst implants in the uterine endometrium. hCG maintains the corpus luteum in early pregnancy, supporting progesterone and estrogen production until placental steroid production takes over. These hormones suppress LH and FSH and help maintain the endometrium; another ovulation normally does not occur.

Statements I, III, and IV are correct. Identical (monozygotic) twins arise when one embryo derived from a single fertilized egg splits. They are genetically very similar, but their bodies are not identical in every detail and their fingerprints differ because ridge patterns also depend on developmental conditions. Fraternal twins arise from two eggs fertilized by two sperm.

Endoderm gives rise to most digestive and respiratory epithelia. Their connective tissues and smooth muscle are primarily mesodermal; the entire organ systems do not arise from endoderm alone. Oral and anal epithelia are exceptions to the general digestive-tract pattern. Somites and the notochord are mesodermal.

D is incorrect: the mammalian zona pellucida is an extracellular glycoprotein coat surrounding the oocyte. In sea urchins, the jelly coat helps trigger the acrosomal reaction, and the sperm binds to the vitelline layer. Cortical-granule exocytosis modifies this layer to form the fertilization envelope.

A is false. In a typical reproductive-age vagina, Lactobacillus species produce lactic acid and help maintain an acidic pH; vaginal cells do not secrete hydrochloric acid as gastric parietal cells do. This acidity supports microbial defense but does not sterilize the vagina. B–E are correct.

The epidermis of skin is derived from ectoderm and the rest of the above are derived from mesoderm.

C is incorrect. Maternal blood occupies the intervillous space, whereas fetal blood remains inside capillaries in chorionic villi. Oxygen, nutrients, carbon dioxide, and wastes cross the placental barrier; the circulations normally remain separate. The umbilical vein carries blood toward the fetus, and the two umbilical arteries carry blood toward the placenta.

Allometric growth means that body parts grow at different relative rates, changing body proportions. Cell differentiation produces specialized cell types. Embryonic induction is signaling by one tissue that influences another tissue’s development. Morphogenesis establishes form and spatial organization. Cytoplasmic determinants are localized molecules that influence cell fate.

The acrosome is a cap on the sperm head containing hydrolytic enzymes. The acrosomal reaction releases these enzymes and helps the sperm penetrate the egg’s extracellular coverings, facilitating subsequent sperm–egg membrane fusion.

Fertilization normally occurs in the ampulla of the uterine (Fallopian) tube. The uterine tube also transports the oocyte or early embryo toward the uterus.

B is incorrect. LH and FSH are secreted by the anterior pituitary. The corpus luteum primarily secretes progesterone and also estrogen. An ovarian follicle contains an oocyte and supporting cells. Ovulation releases a secondary oocyte; normal implantation involves a blastocyst in the uterine endometrium.

The cortical reaction involves exocytosis of cortical granules after egg activation. Their contents elevate and modify the vitelline layer into a fertilization envelope, producing the slow block to polyspermy. Membrane depolarization is the classical fast block. Release of enzymes from the sperm acrosome is the acrosomal reaction; it does not fuse the jelly coat.

Depolarization is the rapid change of the egg membrane potential after sperm–egg fusion. In the classical sea urchin model, it transiently prevents additional sperm from fusing. The sperm nucleus itself does not fuse the plasma membrane. The later cortical reaction provides the slow block to polyspermy; this electrical fast block should not be generalized to human fertilization.

Cleavage produces a morula and subsequently a blastula (a blastocyst in mammals). Gastrulation establishes the major germ layers. Neurulation and organogenesis then establish the early nervous system and other developing organs, although some developmental processes overlap in time. Induction is signaling between tissues and occurs at multiple developmental times; it is not a single fixed stage following gastrulation.

D is correct. The fetal route is placenta → umbilical vein → fetus → umbilical arteries → placenta. Maternal arteries supply blood to the intervillous space, and maternal veins drain it. Substances cross the placental barrier between this maternal blood and fetal capillaries in chorionic villi; there is normally no direct vascular connection between the two circulations.

All four statements are correct. The switch from fetal hemoglobin (HbF) toward adult hemoglobin (HbA) begins around the perinatal period and continues over the first months of life; hemoglobin synthesis does not first begin at birth. Lung expansion lowers pulmonary vascular resistance and pulmonary arterial pressure and increases pulmonary venous return. Left atrial pressure exceeds right atrial pressure, pressing the foramen ovale flap shut functionally. Anatomical fusion occurs later and remains incomplete in some individuals.

Statements I, III, and IV are correct; II is false. Oogenesis begins in the ovary. Meiosis I produces a secondary oocyte and a first polar body, whereas meiosis II normally finishes only after fertilization. Unequal cytokinesis retains most of the cytoplasm in the functional egg. Spermatogenesis has more symmetric meiotic divisions, but mature sperm later shed much of their cytoplasm during spermiogenesis. The ruptured follicle becomes the corpus luteum.

C is incorrect. Progesterone rises as the corpus luteum develops after ovulation, and estrogen has a smaller luteal rise. In the absence of pregnancy, regression of the corpus luteum later lowers both hormones and triggers menstruation. FSH does not continuously increase throughout follicular growth: rising estradiol and inhibin generally suppress it before the midcycle surge.

C is incorrect. The ovulated human secondary oocyte is arrested in metaphase II. Fertilization triggers completion of meiosis II and extrusion of the second polar body. DNA replication and mitotic cleavage follow activation. Cortical and zona changes help prevent polyspermy in humans; the classical sea urchin electrical fast block is not the human mechanism.

A blastula is typically a hollow ball of cells surrounding a blastocoel. A morula is a solid ball of blastomeres. Gastrulation establishes the germ layers. The blastopore is the opening into the archenteron; in the classical comparison, it contributes to the mouth in protostomes and the anus in deuterostomes. Organogenesis forms organ rudiments from the germ layers.

The yolk sac encloses the yolk and supplies nutrients to a bird embryo. The allantois stores nitrogenous wastes, mainly uric acid, and its vessels together with the chorion support gas exchange. The amnion surrounds the embryo with fluid, protecting it from mechanical shock and helping prevent dehydration.

D is incorrect. Sperm are produced in seminiferous tubules. The epididymis supports maturation and is the principal storage site. The vas deferens transports sperm during ejaculation and may also contain stored sperm, so storage alone is not a uniquely false description. Leydig cells produce testosterone, and the prostate contributes fluid containing enzymes such as PSA.

Combined oral contraceptives contain an estrogen and a progestin. They suppress gonadotropin signaling, especially the LH surge, thereby preventing ovulation; progestins also thicken cervical mucus. Progestins are synthetic agents with progesterone-like activity. This contraceptive feedback differs from the sustained high estradiol that triggers the natural preovulatory LH surge.

The midcycle LH surge and smaller FSH surge occur immediately before ovulation. Sustained high estradiol in the late follicular phase produces positive feedback. Progesterone peaks in the midluteal phase; estrogen has a major preovulatory peak and a smaller luteal rise, so the two steroid hormones do not share a single peak.

Tooth enamel is produced by ameloblasts derived from oral ectoderm. Dentin and dental pulp derive from neural crest ectomesenchyme, so a whole tooth should not be described as a single homogeneous tissue. Heart and skeletal muscle are mesodermal. Respiratory and pancreatic glandular epithelia are endodermal.

Amphibian gastrulation involves cell movements that establish ectoderm, mesoderm, and endoderm and form the archenteron and blastopore, including its dorsal lip. The neural tube forms during neurulation. A vague U-shaped appearance is not a defining event common to all gastrulating embryos.

The amnion is an extraembryonic membrane characteristic of amniotes, including reptiles, birds, and mammals. It surrounds the developing embryo and contains amniotic fluid, providing protection against mechanical injury and dehydration. Amphibians are anamniotes and do not form an amnion.

The placenta permits exchange of nutrients, oxygen, carbon dioxide, and wastes across the placental barrier. Maternal blood in the intervillous space and fetal blood in chorionic-villus capillaries normally remain separate. The fetal umbilical vessels connect the fetus to the placenta, not directly to maternal vessels.

B is false. Meiotic divisions in spermatogenesis are relatively symmetric and ordinarily form four haploid spermatids of similar size. During spermiogenesis, much of the cytoplasm is removed in residual bodies, so mature sperm do not retain all the original cytoplasm. FSH and testosterone support spermatogenesis, and LH acts on Leydig cells. Flagellar motility uses ATP supplied by both mitochondrial metabolism and glycolysis.

A is correct. In birds, the allantois stores nitrogenous wastes, especially uric acid, and its vessels combine with the chorion to support gas exchange. The yolk sac supplies nutrients, and the amnion encloses protective fluid. The human allantois is rudimentary and is associated with the urachus and umbilical region; it does not become the placenta. The fetal component of the human placenta derives primarily from the chorion frondosum.