Embryo

An embryo is the earliest developmental stage of a multicellular organism. In species that reproduce sexually, embryonic development begins immediately after fertilisation, when the genetic material of the male sperm cell and the female egg cell combine to form a single-celled zygote. The zygote then undergoes a series of rapid cell divisions and morphological changes that ultimately give rise to the tissues, organs, and body structures of the mature organism. Although the term is often applied specifically to animals, it is also used more broadly for early developmental stages in plants, fungi, and other multicellular organisms.

Etymology

The English word embryo first appears in the fourteenth century and derives from the Medieval Latin embryo, itself taken from Greek embruon, meaning ‘young one’. The Greek term relates to embruos (‘growing within’), formed from en (‘in’) and bruō (‘swell’, ‘be full’). The historically correct Latinised form of the Greek word would be embryum, although embryo has long been standard in English.

Early Stages of Animal Embryonic Development

Animal development begins with fertilisation, which produces a zygote. This single cell contains a diploid nucleus formed from the combined chromosomes of both parents. In most animals, the zygote then undertakes the following major developmental stages: cleavage, blastulation, gastrulation, and organogenesis.
Cleavage
Cleavage refers to rapid mitotic cell divisions that occur without an increase in the overall size of the embryo. Each division generates smaller cells known as blastomeres. At the four-cell stage the embryo resembles a compact cluster of cells, and by the sixteen-cell stage fluid begins to accumulate, forming the blastocoel. This marks the transition to the blastula, or in mammals the blastocyst, which consists of an outer cell layer and an inner cell mass. Prior to embedding into the uterine lining, the mammalian blastocyst undergoes zona hatching to escape the glycoprotein coat surrounding it.
Pre-implantation Conceptus
Before implantation into the uterine wall, the developing mammalian embryo may be referred to as a pre-implantation embryo or pre-implantation conceptus. The term pre-embryo is sometimes used in discussions around early developmental biology and stem-cell research to distinguish this early stage from the later embryo proper.

Gastrulation and the Formation of Germ Layers

Gastrulation is a pivotal phase during which extensive cell migration and tissue rearrangement convert the blastula into an embryo composed of organised germ layers. Animals forming two germ layers, such as cnidarians, are termed diploblastic, whereas most other animal groups, including vertebrates, are triploblastic, forming three germ layers:

• Ectoderm — produces the epidermis, nervous system, and a variety of sensory structures.
• Mesoderm — forms muscles, the vascular and skeletal systems, and connective tissues.
• Endoderm — gives rise to the lining of the digestive tract and much of the respiratory system.

During gastrulation, the embryo undergoes visible structural changes as it folds, invaginates, or elongates to establish the body’s basic axes.

Organogenesis and Later Embryonic Development

Following gastrulation, the embryo enters organogenesis, the stage during which organ rudiments emerge from interactions between the germ layers. Neurulation, for example, involves a specialised region of ectoderm forming the neural tube, the precursor of the brain and spinal cord. As development progresses, cells increasingly specialise, forming tissues characteristic of the fully formed organism.
The duration of the embryonic period varies considerably across species. In humans, the developing organism is termed an embryo until the end of the eighth week after fertilisation, after which it is referred to as a fetus. In many fish species, such as zebrafish, embryonic development is considered complete once specific skeletal elements, such as the cleithrum, appear. In egg-laying animals, the term embryo usually applies only until hatching, while in viviparous species it applies until birth. Because developmental events do not align across all species, embryology is often broadly defined as the study of development from fertilisation through early organ formation.

Plant Embryos and Seed Development

Embryos also form in plants following fertilisation. In flowering plants, a haploid ovule fuses with pollen to create a diploid zygote that develops within the seed. The seed contains three main components: the embryo, the endosperm that stores nutrients, and the seed coat that provides protection.
Zygote Division and Suspensor Formation
The first division of the plant zygote is asymmetric, producing two cells of unequal size: a small apical cell and a larger basal cell.

• The apical cell gives rise to the main structures of the mature plant, including roots, stems, and leaves.
• The basal cell produces the suspensor, a temporary structure that anchors the embryo and channels nutrients from the endosperm.

Embryonic Stages in Plants
The developing plant embryo progresses through several characteristic stages:

• Globular stage — early divisions create spherical embryos in which foundational tissue types emerge:
  • Dermal tissue forming the plant epidermis
  • Ground tissue responsible for photosynthesis, storage, and support
  • Vascular tissue forming xylem and phloem for nutrient and water transport
• Heart stage — one or two cotyledons (embryonic leaves) appear.
• Torpedo stage — meristems, the plant’s lifelong centres of stem-cell activity, become established.

Following embryonic development, many seeds enter dormancy until environmental conditions trigger germination. Once the plant emerges and forms its first true leaves, it becomes a seedling or plantlet.

Embryos in Non-Seed Plants

Plants that reproduce via spores, such as bryophytes and ferns, also generate embryos. In these groups, the embryo develops inside the archegonium on the gametophyte, drawing nutrients from the parent structure during its earliest growth. Although structurally simpler than seed plant embryos, they perform the same fundamental function of establishing the initial body plan of the sporophyte generation.
Embryos, whether in animals or plants, represent the formative stages in the life cycle during which the essential body architecture is established. Through coordinated cellular processes—division, migration, differentiation, and communication—embryos transform from single cells into complex multicellular organisms capable of independent life.