Comparative Anatomy & Developmental Biology

6.1 Comparative Integument (Skin)

The vertebrate integument is always bilayered: an outer epidermis (ectodermal origin, avascular) and an inner dermis (mesodermal origin, vascular, contains collagen, blood vessels, nerves). The two layers interact during embryogenesis to produce class-specific derivatives — scales, feathers, hair — making integumental comparison a reliable indicator of evolutionary grade.

6.1.1 Pisces (Fish)

Fish skin is rich in unicellular mucous glands that lubricate the body and reduce drag. The dermis contains bony scales in most groups. Three main scale types exist: cycloid scales (smooth, concentric growth rings — salmon, carp) are the evolutionarily derived condition; ctenoid scales (comblike posterior margin — perch, bass) are even more derived and provide extra grip; placoid scales (skin teeth, with a pulp cavity + dentine + enameloid cap — sharks and rays) are the most primitive and are homologous to vertebrate teeth. Ganoid scales (diamond-shaped, coated in ganoin — Polypterus, sturgeons) are a fourth archaic type. Pigment cells (chromatophores) reside in the dermis.

6.1.2 Amphibia

Amphibian skin is smooth, moist, and glandular — no scales. The epidermis is thin (important for cutaneous gas exchange). Multicellular mucous glands (keep skin moist) and serous/poison glands (secrete bufotoxins, tetrodotoxin in fire salamanders) are both present. Absence of scales is a derived condition tied to the evolution of cutaneous respiration. The skin must remain moist, restricting most amphibians to humid habitats.

6.1.3 Reptilia

The defining achievement is keratinisation: epidermal scales are made of beta-keratin (not homologous to fish bony scales — they are epidermal, not dermal). Skin is dry and waterproof, enabling terrestrial life. Glands are scarce (femoral pores in some lizards, musk glands in turtles). Periodic ecdysis (moulting) occurs. Crocodilians have an additional layer of osteoderms (dermal bone) beneath the scales.

6.1.4 Aves (Birds)

Feathers are derived from epidermal follicles and are structurally equivalent to reptilian scales (both are epidermal alpha-keratin → beta-keratin structures — birds evolved from theropod dinosaurs). Types: contour feathers (flight + body shape), down feathers (insulation), filoplumes, semiplumes. Birds lack sweat glands; the only significant skin gland is the uropygial (preen) gland at the tail base, secreting oils for waterproofing. Scales persist on the feet and tarsi. The oil-rich preen gland secretion also has antimicrobial properties.

6.1.5 Mammalia

Key mammalian skin derivatives: hair/fur (alpha-keratin filaments from epidermal follicles — functions in thermoregulation, camouflage, sensory); sweat glands (eccrine: thermoregulation; apocrine: scent/pheromone); sebaceous glands (oil secretion, lubricates hair); mammary glands (modified apocrine, define the class — secrete milk). Hooves, claws, nails, and horns are also keratinous epidermal derivatives. Cetaceans (whales) are secondarily hairless; elephants sparsely haired.

6.2 Visceral Arches & Skeletal Origins

In the fish embryo, the pharyngeal wall is supported by a series of cartilaginous visceral arches. The primitive number in vertebrate ancestors is considered to be 6 functional arches (plus a premandibular). During tetrapod evolution, some arches were lost, others were transformed into jaw, hyoid, and middle-ear elements — one of the most elegant examples of structural homology in all of vertebrate anatomy.

6.2.1 The Visceral Arches in Fish

Arch 1 (Mandibular arch): upper element = palatoquadrate (forms upper jaw); lower element = Meckel’s cartilage (forms lower jaw). In higher bony fish, the jaw is reinforced by dermal bones overlaying the cartilages. Arch 2 (Hyoid arch): upper element = hyomandibula (connects jaw to skull in Elasmobranchii); lower element = ceratohyal + basihyal. Arches 3–6 (Branchial arches): support the gills; each arch has epi-, cerato-, hypo-, and basibranchial elements.

6.2.2 Fate in Tetrapods

As vertebrates moved onto land, the gills were lost and the branchial skeleton was repurposed for other functions:

• Arch 1: Palatoquadrate → quadrate bone (reptiles) → incus (middle ear ossicle, mammals). Meckel’s cartilage → articular bone (reptile lower jaw) → malleus (middle ear, mammals). In mammals the entire lower jaw is one bone (dentary).
• Arch 2: Hyomandibula → columella/stapes (amphibian + reptile ear) → stapes (mammal). This accounts for the evolutionary origin of all three mammalian middle-ear ossicles (malleus, incus, stapes) from reptilian jaw/arch bones.
• Arch 3: Forms body of hyoid bone and posterior cornua.
• Arches 4–6: Form laryngeal cartilages (thyroid, cricoid, arytenoids).

6.3 Comparative Alimentary Canal

The gut is lined by endoderm (lining epithelium of digestive tract, liver and pancreatic parenchyma) with mesodermal smooth muscle and serous covering. Major adaptive divergence occurs in stomach type and dentition, reflecting diet.

6.3.1 Dentition Patterns

Homodont dentition: all teeth morphologically similar (fish, reptiles, some amphibians). Heterodont dentition: teeth differentiated into incisors, canines, premolars, and molars (most mammals). The dental formula expresses the number of each tooth type in half the upper jaw / half the lower jaw.

• Human adult (32 teeth): 2123 / 2123 (I C PM M for each half of upper/lower jaw). Total = 2×(2+1+2+3) × 2 = 32.
• Human milk (deciduous, 20 teeth): 2120 / 2120 (no permanent molars).
• Rabbit: 2033 / 1033 = 28 teeth (diastema between incisors and premolars; no canines).
• Dog (carnivore): 3142 / 3143 = 42 teeth (large canines, sectorial carnassials).

Polyphyodont (multiple tooth sets, most vertebrates) vs diphyodont (two sets only — deciduous + permanent, most mammals) vs monophyodont (one set only — toothed whales, some rodents).

Thecodont teeth are set in sockets (mammals, some reptiles); pleurodont teeth are fused to the lateral inner surface of the jaw (most lizards); acrodont teeth sit on the jaw crest (Agama lizards, chameleons).

6.3.2 Stomach Types

Monogastric (simple) stomach: single-chambered, found in humans, dogs, pigs, horses. Ruminant (polygastric) stomach: four-chambered in cattle, sheep, goats — the only vertebrates with a true four-chambered stomach in the digestive sense (not to be confused with the four-chambered heart of birds and mammals). The four chambers are:

1. Rumen (paunch) — largest; microbial fermentation of cellulose; bolus is regurgitated for remastication (cud-chewing).
2. Reticulum (honeycomb stomach) — filters food; hardware disease (foreign metal objects collect here).
3. Omasum (manyplies/psalterium) — leaf-like folds absorb water and volatile fatty acids.
4. Abomasum (true stomach) — glandular; HCl + pepsin digestion.

Avian stomach is two-part: the crop (diverticulum of oesophagus; stores and softens food; produces “pigeon milk” in columbids) and the two-part stomach proper: proventriculus (glandular, secretes HCl + pepsin) + gizzard/ventriculus (muscular, grinds hard food; replaces teeth). Raptors and owls regurgitate undigested bones + fur as pellets from the gizzard.

6.4 Comparative Respiratory System

Oxygen extraction evolved from aqueous to aerial environments, requiring progressively more sophisticated gas-exchange surfaces. The respiratory organ must maximise surface area, minimise diffusion distance, and maintain a concentration gradient.

6.4.1 Pisces — Gills

Gills are richly vascularised lamellate structures on the branchial arches. In bony fish, 4 pairs of gills are housed in a opercular chamber; sharks have 5–7 exposed gill slits. The critical adaptation is countercurrent exchange: water flows over gill lamellae in the opposite direction to blood flow, maintaining a diffusion gradient along the entire exchange surface (up to 90% O&sub2; extraction vs ~50% in concurrent flow). Lungfish (Protopterus, Lepidosiren, Neoceratodus) also possess lungs (modified swim bladder) for aerial breathing during drought.

6.4.2 Amphibia — Multiple Routes

Adult amphibians use up to three simultaneous routes of gas exchange: (i) Cutaneous respiration through moist skin (accounts for ~60% of O&sub2; uptake in frogs at rest); (ii) Buccopharyngeal (mouth floor and pharyngeal mucosa); (iii) Pulmonary — simple sac-like lungs without alveoli, ventilated by positive-pressure buccal pumping. Larvae breathe via external gills (plumose), which are resorbed at metamorphosis. No diaphragm; frogs gulp air with mouth closed.

6.4.3 Reptilia — Lungs Alone

Reptiles are the first vertebrates relying solely on lungs. Lungs are more folded (septate) than amphibian sacs, increasing surface area. Negative-pressure ventilation via rib muscles (no diaphragm in most). Crocodilians have a unique hepatic-piston diaphragm (liver pulled back by diaphragmaticus muscle). Turtles ventilate by moving the forelimbs (no rib cage expansion possible). Skin is waterproof — no cutaneous exchange.

6.4.4 Aves — Air Sacs and Parabronchi (most efficient)

The avian lung is the most efficient vertebrate respiratory system. Key features:

• 9 air sacs (1 interclavicular + 2 anterior thoracic + 2 posterior thoracic + 2 cervical + 2 abdominal) connected to rigid lungs.
• Parabronchi (air capillaries) replace alveoli; gas exchange occurs in tiny air capillaries woven among blood capillaries in a crosscurrent arrangement.
• Unidirectional (through-flow) ventilation: air always flows in the same direction through the parabronchi (posterior-sac → lung → anterior sac) during both inhalation and exhalation — requires two respiratory cycles per breath.
• No stale air remains in lungs (unlike tidal ventilation of mammals). O&sub2; extraction ~90%.
• Air sacs also reduce body density (for flight) and cool the body (no sweat glands).

6.4.5 Mammalia — Alveolar Lungs + Diaphragm

Mammalian lungs have millions of alveoli (blind-ended sacs, 70–80 m² in humans) for gas exchange. A complete muscular diaphragm (unique to mammals) drives negative-pressure ventilation. The respiratory tree: trachea → bronchi → bronchioles → respiratory bronchioles → alveolar ducts → alveoli. Surfactant (DPPC) from type-II pneumocytes reduces surface tension. Breathing is tidal (bidirectional); residual volume (~1.2 L in humans) means gas exchange is less efficient than in birds.

6.5 Comparative Heart & Circulation

The vertebrate heart evolved from a simple contractile tube (single circuit) to a four-chambered double pump (two circuits). The primitive vertebrate heart has four serial chambers — sinus venosus → atrium → ventricle → conus arteriosus. Evolution involved partial to complete subdivision of these chambers and the great vessels.

6.5.1 Pisces (Fish) — 2-Chambered Heart

1 atrium + 1 ventricle (plus sinus venosus anteriorly and conus arteriosus/bulbus arteriosus posteriorly). Single circulation: blood follows one loop: body → heart → gills (oxygenation) → body. Blood is always mixed but passes through the gill capillaries before the systemic capillaries. The heart receives only deoxygenated blood. Low systemic pressure (gill resistance in series).

6.5.2 Amphibia — 3-Chambered Heart

2 atria + 1 ventricle. The right atrium receives deoxygenated blood (from body); the left atrium receives oxygenated blood (from lungs + skin). Both drain into a single undivided ventricle → theoretically mixed blood, but the spiral valve in the conus arteriosus and the trabeculae in the ventricle wall limit mixing. Double but incomplete circulation. Sinus venosus still present. Truncus arteriosus divides into pulmonary and systemic arches via the spiral valve.

6.5.3 Reptilia — Incompletely 4-Chambered

2 atria + 2 ventricles, but the interventricular septum is incomplete (Foramen of Panizza) in most reptiles. Crocodilians are the exception: they have a complete four-chambered heart (like birds/mammals), but a connection (foramen of Panizza) between the left and right aortic arches allows mixing during diving. Blood mixing possible in non-crocodilian reptiles; useful for diving (shunting blood away from lungs). Sinus venosus and conus arteriosus mostly incorporated into atrial and ventricular walls.

6.5.4 Aves & Mammalia — Complete 4-Chambered

2 atria + 2 ventricles, with a complete interventricular septum. Double, complete circulation: no mixing of oxygenated and deoxygenated blood. Right side (pulmonary circuit): deoxygenated blood → lungs. Left side (systemic circuit): oxygenated blood → body. Left ventricle wall is ~3× thicker than right (higher systemic pressure). Sinus venosus and conus arteriosus entirely incorporated. Aortic arch differences: Birds retain only the right systemic aortic arch; mammals retain only the left systemic aortic arch. Both birds and mammals lose the other arch (opposite sides) independently — a convergent solution.

Abbreviations: A = atrium; V = ventricle; SV = sinus venosus; CA = conus arteriosus.

6.6 Comparative Excretory System — Kidney Evolution

The vertebrate kidney evolved through three successive types: pronephros → mesonephros → metanephros. Each represents an improvement in filtration surface, tubule length (for reabsorption), and positional efficiency. The shift tracks the body’s move from aquatic to terrestrial life and the need for progressively more concentrated urine.

6.6.1 Pronephros

The pronephros (pro = first) is the most anterior and most primitive kidney. Present transiently in the embryos of all vertebrates. Functional (excretes nitrogenous waste) in larvae of the most primitive fish (Petromyzon ammocoetes larvae) and larval amphibia. Tubules open into the archinephric duct (pronephric duct), which runs to the cloaca. Filtration occurs through external glomeruli projecting into the coelom (coelomostome → tubule). Extremely inefficient; soon replaced.

6.6.2 Mesonephros

The mesonephros (meso = middle) is the functional kidney in adult fish (except hagfish/lamprey) and adult amphibians. In amniotes (reptiles, birds, mammals), it functions only transiently during embryonic life. Internal glomeruli enclosed in Bowman’s capsule (no coelomostome). Tubules drain into the mesonephric (Wolffian) duct, which leads to the cloaca.

Wolffian duct fate: In males, the mesonephric duct becomes the epididymis + vas deferens + ejaculatory duct. In females, it largely degenerates (vestigial Gartner’s duct).

6.6.3 Metanephros

The metanephros (meta = after) is the definitive adult kidney of all amniotes (reptiles, birds, mammals). Arises from a ureteric bud (outgrowth of the Wolffian duct) + metanephrogenic blastema (mesoderm). Has the most tubules and highest filtration area. Drains via a ureter to the urinary bladder (mammals/most reptiles) or cloaca (birds/reptiles). Mammals can produce concentrated urine due to the loop of Henlé in the nephron — absent in fish and amphibia.

6.6.4 Wolffian and Müllerian Duct Fates

Both ducts develop alongside the mesonephros in all vertebrate embryos. Sexual differentiation determines which duct persists:

• Males: Wolffian duct → epididymis, vas deferens, seminal vesicle, ejaculatory duct (testosterone-dependent maintenance). Müllerian duct degenerates due to Anti-Müllerian Hormone (AMH) secreted by Sertoli cells of the developing testis.
• Females: Müllerian duct → fallopian tubes (uterine tubes), uterus, upper vagina. Wolffian duct degenerates (absence of testosterone). In the absence of AMH and testosterone, Müllerian derivatives form = default female pathway.

6.7 Comparative Brain & Nervous System

The vertebrate brain derives from three primary vesicles of the embryonic neural tube: prosencephalon (forebrain), mesencephalon (midbrain), and rhombencephalon (hindbrain). These further subdivide into five secondary vesicles, each giving rise to named brain regions.

6.7.1 Forebrain (Prosencephalon)

Telencephalon: cerebral hemispheres (cerebrum) + olfactory bulbs/lobes. In fish, the cerebrum is primarily olfactory (smell-brain). In mammals, the cerebral cortex becomes massively expanded and folded (neocortex) and dominates all sensory and motor integration. Diencephalon: thalamus (relay centre), hypothalamus (homeostasis centre — thermoregulation, hunger, circadian rhythm, hormonal control via pituitary), pineal body (melatonin), optic chiasma.

6.7.2 Midbrain (Mesencephalon)

In lower vertebrates (fish, amphibia), the optic lobes (corpora bigemina) are the dominant midbrain structure — primary visual processing centre. In mammals, replaced by corpora quadrigemina (superior + inferior colliculi — reflex coordination of vision and hearing); visual processing moves to the occipital cortex.

6.7.3 Hindbrain (Rhombencephalon)

Metencephalon: cerebellum (coordination of voluntary movement, balance, muscle tone) + pons (relay between cerebellum and cerebral cortex). Myelencephalon: medulla oblongata (vital reflex centres: cardiac, respiratory, vasomotor). The cerebellum is largest (relative to brain mass) in birds, where precise flight coordination demands maximum input.

6.8 Embryology — Cleavage, Gastrulation, Organogenesis

Embryonic development begins at fertilisation. The fertilised egg (zygote) undergoes a series of mitotic divisions called cleavage to produce a multicellular morula, then a fluid-filled blastula (blastocyst in mammals). The degree of yolk present in the egg determines both the pattern and completeness of cleavage.

6.8.1 Types of Eggs (Yolk distribution)

• Isolecithal (microlecithal): little, uniformly distributed yolk. Sea urchin, lancelet, placental mammals (yolk functions replaced by placenta).
• Mesolecithal (moderately yolky): moderate yolk concentrated at vegetal pole. Frogs, most amphibia.
• Telolecithal (macrolecithal / megalecithal): massive yolk at vegetal pole; cytoplasm at animal pole. Reptiles, birds, sharks.
• Centrolecithal: yolk in centre, cytoplasm peripheral. Insects (e.g., Drosophila).

6.8.2 Cleavage Patterns

Cleavage divisions are rapid mitoses (no cell growth between divisions). Pattern determined by yolk amount and distribution:

• Holoblastic (complete): entire egg divides. Seen in isolecithal and mesolecithal eggs. Sub-types: equal holoblastic (sea urchin — all blastomeres equal) and unequal holoblastic (frog — vegetal pole cells larger = macromeres; animal pole = micromeres).
• Meroblastic (incomplete): only the cytoplasm-rich disc at the animal pole cleaves; yolk mass does not divide. Sub-types: discoidal meroblastic (reptiles, birds, sharks — cleavage restricted to germinal disc); superficial meroblastic (insects — cleavage only at cortical ring around central yolk).

Additional axes of classification:

• Radial cleavage: planes at right angles; blastomeres stack directly above each other. Characteristic of deuterostomes (echinoderms, chordates including frog, amphioxus). Associated with indeterminate development (each blastomere retains totipotency).
• Spiral cleavage: upper tier of cells rotates 45° relative to lower tier. Characteristic of protostomes (annelids, molluscs, platyhelminthes). Associated with determinate development (fate fixed early; removing one cell = incomplete larva).

6.8.3 Blastula Stage

After cleavage, cells arrange into a hollow ball: the blastula. The fluid-filled cavity is the blastocoel. In the frog, the blastocoel is displaced to the animal hemisphere because the yolk-laden vegetal cells are too large to move inward. In the chick, the equivalent is a flat disc (blastoderm) resting on the yolk. In mammals, the blastula is a blastocyst: outer trophoblast (future placenta) + inner cell mass (ICM = embryoblast, future embryo).

6.8.4 Gastrulation

Gastrulation converts the blastula into the gastrula, establishing the three primary germ layers: ectoderm, mesoderm, and endoderm. The mechanisms include:

• Invagination: infolding of the vegetal pole cells inward (sea urchin, amphioxus).
• Involution: rolling of surface cells inward over a rim (amphibia).
• Ingression: individual cells migrate inward from the surface (sea urchin primary mesenchyme).
• Epiboly: spreading of thin ectoderm cells over the surface, covering the deeper endodermal cells (prominent in frog and teleost fish gastrulation).
• Delamination: splitting of a layer to produce two layers (occurs in some avian and mammalian stages).

The inpocketing creates the archenteron (primitive gut); its opening to the exterior is the blastopore. In deuterostomes (echinoderms, chordates), the blastopore becomes the anus (mouth forms secondarily). In protostomes (annelids, arthropods, molluscs), the blastopore becomes the mouth.

6.8.5 Neurulation and Organogenesis

After gastrulation, the notochord (mesoderm) induces the overlying ectoderm to thicken into the neural plate. The plate folds to form the neural tube (future central nervous system) in a process called neurulation; the resulting embryonic stage is a neurula. Cells at the edges of the neural folds break away as the neural crest — a pluripotent migratory population giving rise to peripheral ganglia, Schwann cells, melanocytes, adrenal medulla, craniofacial cartilage, and more (sometimes called the “fourth germ layer”).

6.8.6 Spemann’s Organiser (1924)

Hans Spemann and Hilde Mangold (1924, Nobel 1935) performed a classic experiment: they transplanted the dorsal lip of the blastopore of a newt gastrula (the “organiser”) to the ventral side of another embryo. The result was a second complete body axis — a Siamese-twin embryo. The organiser induces surrounding cells to adopt neural and axial fates, establishing the anterior–posterior axis. The molecular basis involves secreted inhibitors (Chordin, Noggin, Follistatin) that block BMP-4 signalling, allowing neural identity in the overlying ectoderm (default neural induction model).

6.9 Frog Development & Germ-Layer Derivatives

The frog (Rana) is the classic vertebrate embryology model due to the accessibility of eggs and the moderate yolk making all stages visible externally.

6.9.1 Frog Egg and Cleavage

Frog eggs are mesolecithal: moderate yolk concentrated at the vegetal pole (yolk-laden macromeres = white/pale); the animal pole (pigmented, yolk-poor = micromeres) faces upward in pond water. Cleavage is holoblastic and unequal: 1st and 2nd cleavages are vertical (meridional), passing through animal and vegetal poles; 3rd cleavage is horizontal (equatorial) but displaced to the animal pole, producing 4 smaller micromeres on top and 4 larger macromeres below. The blastomere inequality increases with successive divisions.

6.9.2 Blastula

The frog blastula (blastocoel displaced toward the animal pole) has a sub-germinal cavity (blastocoel) not at the centre but in the animal hemisphere, because the large vegetal macromeres cannot move inward. The embryo is about 3–4 mm, same as the unfertilised egg (cleavage divisions do not increase total cell mass).

6.9.3 Gastrulation in the Frog (Epiboly + Involution + Yolk plug)

Gastrulation begins with the formation of the dorsal lip of the blastopore on the future dorsal side of the embryo (grey crescent region). Cells involute over the dorsal lip, forming ectoderm (outer), mesoderm (middle as cells roll in), and endoderm (inner — the large yolk cells). Epiboly carries the animal-pole ectoderm over the entire surface. As gastrulation proceeds, the blastopore is visible as a ring around a mass of yolk cells called the yolk plug (endoderm not yet fully internalised — visible at the vegetal surface). The archenteron expands as the blastocoel shrinks. At the end of gastrulation, the yolk plug disappears as it is covered by ectoderm.

6.9.4 Germ-Layer Derivatives

The three germ layers give rise to all adult organs. The standard derivation list is one of the highest-yield items in any embryology exam:

6.10 Quick-Reference Tables

End of Chapter 6 · Comparative Anatomy & Developmental Biology. HPRCA-pat. indicates HPRCA / state-TGT pattern questions; literal past-paper items will be flagged with year when official papers are sourced.