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A picture of a human embryo at 5 weeks of development.
A picture of a human embryo at 7 weeks of development.

The embryo is a sexually reproducing organism in the early stages of its development. In humans, the term is traditionally reserved for the first two months of development. After that point, the term “embryo” is replaced by the term “fetus,” which then applies until birth. At the beginning of the individual’s development, the entity is a single cell. After two months, it has limbs, distinct fingers and toes, internal development, and countless cells. So the term “embryo” applies to an individual throughout a vast range of developmental change.[1]

Human embryo about fifteen days old. Brain and heart represented from right side. Digestive tube and yolk sac in median section.

An embryologist is a person who studies embryos (Greek: έμβρυον, embruon), the formation and development of which is known as embryogenesis. Embryology is the branch of developmental biology that studies the beginning and early growth of sexually reproducing organisms. This scientific specialty focuses on the formation and development of embryos.


Main Article: Embryogenesis

A single sperm penetrates the mother's egg cell, and the resulting cell is called a zygote. The zygote contains all of the genetic information (DNA) necessary to become a child. Half of the genetic information comes from the mother’s egg and half from the father’s sperm. The zygote spends the next few days traveling down the Fallopian tube and divides to form a ball of cells.[2]

Chicken blastocyst (day 7)

The zygote continues to divide, creating an inner group of cells with an outer shell. This stage is called a blastocyst. The inner group of cells will become the embryo, while the outer group of cells will become the membranes that nourish and protect it. In humans, the blastocyst reaches the womb (uterus) around day 5, and implants into the uterine wall on about day 6. At this point in the mother's menstrual cycle, the lining of the uterus has grown and is ready to support a fetus. The blastocyst sticks tightly to the lining, where it receives nourishment via the mother's bloodstream.[2]

The cells of the embryo now multiply and begin to take on specific functions. This process is called differentiation. It leads to the various cell types that make up a human being (such as blood cells, kidney cells, and nerve cells). There is rapid growth, and the baby's main external features begin to take form. It is during this critical period (most of the first trimester) that the growing baby is most susceptible to damage.[2]

Summary of Developmental Timecourse

Stage Week after fertilization Days after fertilization Event
Pregenesis: development of parents 4th week development of parents 24 Parents' primordial germ cells (PGCs) begin their migration to parents' gonads
Blastogenesis 1st week, embryo is unilaminar 1 Fertilization
1.5-3 1st cleavages, move to uterus
5 Free blastocyst in uterus
5-6 Hatching, start implantation
2nd week, embryo is bilaminar 7-12 Fully implanted
13 Primary stem villi and primitive streak appear
3rd week, embryo is trilaminar 16 Gastrulation begins, notochord forms
18 Primitive pit, neural plate, neural groove
20 First somites, primitive heart tube
4th week 22 Neural folds fuse, pulmonary primordium
24 PGCs begin migration, Cranial neuropore closes, optic vesicles and pit form
26 Caudal neuropore closes, arm limb buds
28 Leg limb buds, more brain, eye/ear devel.
Organogenesis 5th - 8th weeks 29-56
Phenogenesis 9th - 38th weeks

See Also


  1. 1.0 1.1 Monitoring Stem Cell Research: Appendix A (Notes on Early Human Development) by The President's Council on Bioethics.
  2. 2.0 2.1 2.2 Fetal Development by U.S. National Library of Medicine.