Gigantism is caused by excess growth hormone (GH) secretion from the anterior pituitary gland during childhood, before epiphyseal plates close. Down's, Turner's, and Cri-du-chat syndromes are chromosomal abnormalities, not glandular.

Secondary growth (increase in girth) is produced by lateral meristems — vascular cambium (produces secondary xylem and phloem) and cork cambium (produces bark). Apical meristems cause primary growth (length increase).

Metamorphosis is the dramatic transformation from larval form to adult form. In amphibians (frogs) it involves replacement of aquatic larval structures (gills, tail) with terrestrial adult structures (lungs, limbs) under hormonal control.

Blue light (detected by cryptochromes and phototropins) promotes cell division and inhibits cell elongation in plants. It also controls stomatal opening, phototropism, and chloroplast movement.

Lateral meristems (vascular cambium and cork cambium) are present in dicots and gymnosperms, enabling secondary growth (increase in girth). Monocots generally lack lateral meristems and do not show secondary growth.

Regenerative ability is generally greater in invertebrates than vertebrates. Sponges, hydra, and planaria have extraordinary regeneration; echinoderms and annelids moderate; amphibians (salamanders) among vertebrates are best; mammals have least.

Plant growth involves three stages: cell division (mitosis in meristems producing new cells), cell elongation (vacuolation and water uptake enlarging cells), and cell maturation/differentiation (specialisation into different cell types).

Starfish can regenerate lost arms — and remarkably, in many species a severed arm can regenerate a complete new body. This powerful regenerative ability is used as a means of asexual reproduction in some starfish species.

Neoblasts are the pluripotent stem cells found in flatworms (planaria) that are responsible for their extraordinary regenerative ability. When a planarian is cut, neoblasts migrate to the wound and differentiate into all missing cell types.

Regeneration is the biological capacity to regrow, repair, or restore lost or damaged body parts. The degree of regenerative ability varies enormously among organisms — from complete regeneration in planaria to limited healing in humans.

The blastula is a hollow sphere of cells (blastomeres) enclosing a fluid-filled cavity called the blastocoel. It forms after sufficient cleavage divisions have occurred following fertilization.

The ectoderm (outer germ layer) differentiates into: nervous system (brain, spinal cord, nerves via neural tube), skin epidermis (and derivatives — hair, nails, glands), and sensory organs (eye lens, inner ear, olfactory epithelium).

Muscle cells are specialised to express the contractile proteins actin and myosin in large quantities. During differentiation, muscle precursor cells (myoblasts) fuse and activate genes for actin, myosin, troponin, and tropomyosin.

The mesoderm (middle germ layer) gives rise to muscles, bones, cartilage, the circulatory system, kidneys, gonads, adrenal cortex, connective tissues, and the dermis of skin.

Microcephaly is a developmental abnormality where the brain fails to develop normally, resulting in an abnormally small head (and often brain). It can result from genetic mutations, Zika virus infection, radiation, or malnutrition during pregnancy.

Gastrulation is defined by extensive coordinated cell movements — invagination, ingression, epiboly, and convergent extension — that establish the three primary germ layers and the basic body plan of the embryo.

The calcareous eggshell is secreted by the uterus (shell gland/uterine shell gland) as the egg passes through it. Different sections of the oviduct add different components — infundibulum (fertilization), magnum (albumin), uterus (shell).

During early gastrulation in the chick, the blastoderm separates into an upper epiblast (gives rise to embryo proper) and a lower hypoblast (gives rise to extra-embryonic membranes). This occurs before the primitive streak forms.

Chicken embryos incubated at 37-38°C complete development and hatch on day 21. This is the standard incubation period for domestic chickens (Gallus gallus domesticus).

As the fertilised egg passes through the female reproductive tract (oviduct), it receives accessory coverings: albumin (egg white), shell membranes, and finally the calcareous shell secreted in the uterus (shell gland).

Regeneration is the basis of vegetative propagation in plants. Plants can regenerate from cuttings, grafts, and tissue culture because plant cells are totipotent — a single cell can grow into a complete plant.

Gerontology is the multidisciplinary study of aging processes — their biological mechanisms, psychological effects, and social implications. Geriatrics is the clinical medical specialty treating age-related diseases.

Both lizards (reptiles) and amphibian larva/salamanders can regenerate their tails. Salamanders can also regenerate limbs, heart tissue, and parts of eyes and brain — making them important models for regeneration research.

Regeneration is the ability to regrow lost or damaged body structures. It varies widely — from simple wound healing in all animals to complete limb regeneration in salamanders and whole-body regeneration from fragments in hydra and planaria.

Aging (senescence) is an inevitable, progressive deterioration of physiological functions with time. Despite medical advances, aging cannot be stopped — it is a universal feature of all living organisms programmed into their biology.

Embryonic induction is the process by which one group of cells (inducer/organizer) directs the development of neighbouring cells (responders). Spemann showed the dorsal lip induces neural tube formation — the primary induction.

Hammerling's classic experiments with Acetabularia showed that the nucleus (in the rhizoid) controls cap formation — it produces morphogenetic substances (mRNA) that determine which species-specific cap develops.

The grey crescent cytoplasm contains cortical determinants (morphogenic factors) essential for development. Spemann demonstrated that the grey crescent cytoplasm (not nucleus) is needed for normal embryonic development — it becomes the dorsal lip.

The genome (complete set of genetic information in the nucleus) of the zygote contains all instructions needed for development. Gene expression is differentially regulated to produce the different cell types of the organism.

Hensen's node (primitive node) in the chick embryo is the organiser region equivalent to the dorsal lip of the blastopore in frogs. It directs the formation of the notochord and organises the dorsal mesoderm into somites.

The blastula stage in chick development is characterised by the presence of the blastocoel (segmentation cavity/subgerminal space) between the blastoderm (disc of cells) and the yolk mass below.

The epiblast in chick development gives rise to all three germ layers — ectoderm, mesoderm, and endoderm. The hypoblast contributes mainly to extra-embryonic endoderm (yolk sac). So epiblast is primarily presumptive ectoderm and mesoderm.

In the developing chick, after gastrulation the cavity between the developing endoderm/hypoblast and the yolk becomes the gastrocoel (subgerminal cavity). The blastocoel is the original cavity between epiblast and hypoblast.

The blastoderm is the disc of cells (blastodisc) that forms above the blastocoel (sub-germinal space) in telolecithal eggs like the chick. It consists of the epiblast and hypoblast layers after gastrulation begins.

Cleavage is the series of rapid mitotic divisions of the fertilised egg without cell growth. In the chick, cleavage is meroblastic (incomplete) — only the blastodisc (not the yolk) divides, forming the blastoderm.

Leaf primordia and lateral bud primordia arise from the flanks of the shoot apical meristem. The SAM continuously generates these primordia in a characteristic phyllotactic pattern (spiral, alternate, opposite).

Gastrulation is the developmental stage during which the three primary germ layers (ectoderm, mesoderm, endoderm) are established through cell movements. It follows blastulation and precedes organogenesis.

Lateral root primordia (young lateral roots) originate from the pericycle — a single cell layer just inside the endodermis of the main root. They push through the cortex and endodermis to emerge as lateral roots.

Blue light (400-500 nm) detected by cryptochromes promotes cell division (splitting of chloroplasts, nuclear division) but retards cell elongation — this is why blue light produces compact, stocky plants compared to red light.

Ultraviolet (UV) light inhibits cell elongation by causing crosslinking of proteins and DNA damage, and by stimulating production of inhibitory compounds. UV also causes mutations that can affect growth.

Leaf primordia are groups of cells at the shoot apical meristem that are committed (determined) to develop into leaves, lateral shoots, or flower organs. They are the first recognisable structures of future organs.

Red light promotes cell elongation in etiolated (dark-grown) seedlings by activating phytochrome. It also suppresses shade-avoidance responses. Blue light inhibits elongation by activating cryptochromes.

Most plants stop growing and may die at temperatures above 35-40°C because enzyme denaturation occurs, membrane integrity is lost, and metabolic processes fail. Optimal growth temperatures are usually 20-30°C for most plants.

Cork cambium (phellogen) is a lateral meristem that produces cork cells (phellem) outward and phelloderm inward, forming the outer bark (periderm) of woody stems. It replaces the epidermis in secondary growth.

Red light (660 nm) activates phytochrome (converting Pr to Pfr), which promotes photomorphogenesis including cell differentiation, leaf expansion, chloroplast development, and inhibition of stem elongation.

External (environmental) factors affecting plant growth include light (photosynthesis, photomorphogenesis), temperature (enzyme activity rates), oxygen (aerobic respiration), water, minerals, and CO₂.

In vascular plants, growth is localised at meristems — specialised regions of actively dividing cells. Apical meristems drive primary (length) growth; vascular and cork cambium drive secondary (girth) growth.

Growth is the permanent, irreversible increase in size and dry mass of an organism. It involves cell division, cell enlargement, and differentiation. Development is the broader process including all changes in form and function.

Meristems (growth regions) are located at root tips (root apical meristems) and shoot tips (shoot apical meristems) and also in nodes (intercalary meristems in grasses) and along stems (lateral meristems/cambium).

Plant growth is classified as primary (from apical meristems — increases length) and secondary (from lateral meristems/cambium — increases girth/width). Together they produce the characteristic form of woody plants.

Regeneration is the ability to regrow or replace lost or damaged body parts. It ranges from simple wound healing in all animals to complete limb regeneration in salamanders and whole-body regeneration from fragments in planaria and hydra.

Apical meristems (at root and shoot tips) are responsible for primary growth — increasing the length of roots and stems. Lateral meristems (cambium) increase girth. Intercalary meristems increase internode length in grasses.

Both chromosomal abnormalities (errors in chromosome number — trisomy, monosomy — or structure — deletions, translocations) and gene mutations can occur during gamete formation (meiosis), causing developmental disorders.

The nucleus of the zygote (and of every somatic cell) contains the complete genome — all the genetic information needed to develop into a complete individual. Each nucleus has a full copy of the organism's DNA.

Embryology (developmental biology) is the study of how organisms develop from a single cell (zygote) through successive stages to form a complex multicellular organism, including all structural and functional changes.

In Acetabularia (a giant unicellular alga), if the cap is removed, the nucleus in the rhizoid produces substances that cause a new cap to regenerate. This demonstrates nuclear control of regeneration and differentiation.

Sponges and coelenterates (Hydra, jellyfish) have exceptional regenerative abilities. Hydra can regenerate a complete organism from a small fragment. Sponges dissociated into individual cells can reform into complete sponges.

In many plants (especially angiosperms), living vegetative cells are totipotent — they retain the ability to regenerate an entire plant. This is the basis of tissue culture, cutting propagation, and natural vegetative reproduction.

Lizards can regenerate their tails after autotomy (voluntary tail shedding to escape predators). However, the regenerated tail is made of cartilage rather than bone and lacks the original scale pattern.

Regeneration is the biological process by which organisms regrow lost or damaged body parts or reconstruct an entire organism from a fragment. It ranges from simple wound healing to complete regrowth of limbs (as in salamanders).

All organisms have aging genes (gerontogenes) that control the rate of aging and maximum life span. These include genes like p53 (tumour suppressor), telomerase, and various DNA repair genes that influence longevity.

Every organism progresses through a life cycle: growth (increase in size and complexity), maturation (reaching reproductive capability), and senescence (aging — gradual deterioration of functions), ultimately ending in death.

Gerontology is the scientific study of aging and the problems associated with it — biological, psychological, and social aspects of growing old. Geriatrics is the medical specialty treating the elderly.

Amoeboid macrophages phagocytose the cellular debris produced during histolysis of the gills and tail in metamorphosing tadpoles. These phagocytic cells clean up the autolysed tissue, allowing reabsorption.

During frog metamorphosis, the hindlimbs develop and increase in size before the forelimbs. The hindlimbs appear first as small buds, elongate, and differentiate into thigh, leg, and foot before the forelimbs emerge.

The first pharyngeal (gill) pouch in vertebrate embryos develops into the tympanic cavity (middle ear cavity) and Eustachian tube. The eardrum (tympanic membrane) forms between the first pharyngeal cleft and pouch.

During frog metamorphosis, the tadpole tail (with its dorsal and ventral fin folds) is completely reabsorbed through histolysis (cell death and autophagy mediated by thyroxine). The tail disappears as the adult frog develops limbs.

The proctodaeum is the posterior ectodermal invagination that forms the hindgut/cloaca. The stomodaeum is the anterior ectodermal invagination forming the mouth. Both are formed from ectoderm, not the archenteron.

As the tail elongates during frog tadpole development, the tail somites differentiate into v-shaped myomeres (muscle segments). These segmental muscles power the swimming movements of the tadpole.

The hypomere (lateral plate mesoderm) splits into two layers: the outer somatic (parietal) layer adjacent to ectoderm, and the inner splanchnic (visceral) layer adjacent to endoderm. The space between them becomes the coelom.

During gastrulation, the mesoderm forms as the third (middle) germ layer between the outer ectoderm and inner endoderm. It gives rise to muscles, bones, the circulatory system, kidneys, and gonads.

The ectoderm gives rise to the nervous system (brain, spinal cord, nerves), epidermis (outer skin), sensory organs (eyes, ears), lining of mouth and cloaca, and the enamel of teeth.

The neural groove forms when the edges (neural folds) of the neural plate thicken, rise up, and fold inward. When the neural folds fuse along the midline, they form the neural tube (precursor of brain and spinal cord).

During gastrulation in frogs, the large yolk-laden cells of the vegetal pole are visible through the blastopore as the yolk plug — a mass of yolk cells that temporarily blocks the blastopore opening.

Pigmented cells from the animal pole of the frog blastula overgrow the yolk cells during gastrulation in a process called epiboly. The pigmented animal cells spread downward and around the vegetal yolk mass.

During gastrulation in the frog, the blastopore forms below the equator and the animal pole rotates approximately 120° relative to its original position as the yolk-laden vegetal cells are overgrown by animal cells (epiboly).

During gastrulation, as the archenteron (primitive gut/gastrocoel) expands due to invagination of cells, the blastocoel is progressively eliminated (obliterated) and eventually disappears completely.

Cleavage is the series of rapid mitotic cell divisions that occur after fertilization without cell growth. The zygote divides into progressively smaller cells (blastomeres), forming the morula and then the blastula.

The morula (Latin: mulberry) is a solid ball of cells formed after several cleavage divisions of the zygote, before a cavity (blastocoel) forms. It resembles a mulberry and consists of 16-32 blastomeres.

The blastula is a hollow ball of cells (blastomeres) enclosing a fluid-filled cavity called the blastocoel. It forms after repeated cleavage divisions of the zygote. In frogs it is called the blastula; in mammals the blastocyst.

Fertilization is the fusion of male (sperm) and female (egg/ovum) gametes. This union forms a diploid zygote containing the complete genetic information needed to develop into a new organism.

Telolecithal eggs contain a considerable amount of yolk concentrated at the vegetal pole. The weight of yolk pulls it to the bottom. Examples include frog (moderately telolecithal) and bird eggs (extremely telolecithal).

Embryology is the branch of biology that studies the development of embryos from fertilization through the major stages — cleavage, gastrulation, organogenesis — until birth or hatching.

Vertebrate eggs are classified based on the amount and distribution of yolk: isolecithal (little yolk, evenly distributed), telolecithal (large yolk concentrated at vegetal pole), and centrolecithal (yolk in centre).

Isolecithal eggs have very little yolk distributed uniformly throughout the cytoplasm. Examples include sea urchin and mammalian eggs. The even yolk distribution allows equal (holoblastic) cleavage.