General knowledge forms a desirable starting point for the study of special embryology to be taken up later. As comparative anatomy has proven to be the best basis for the later study of anatomy, so comparative embryology forms a proper basis for the study of special embryology.
— Aute Richards, Outline of Comparative Embryology

Embryology: the study of growth and development of the “embryo” and “fetus” from “fertilization” of the “ovum” until birth. (OxfordMed) How the single-celled fertilized egg arrives at the multicellular condition characteristic of the fully formed, but undifferentiated, young “organism” in which the “organ” systems are established. (Richards, xiii)

Begins with week 3 and extends through week 8. During this time of rapid growth and intensive "cell differentiation," the organs and major systems of the body form. By the end of the embryonic period, the embryo has grown from a cluster of a few hundred "cells" no bigger than the head of a pin to over an inch in length, now weighing about an ounce. (Hockenbury, 356) “Neurogenesis” (the process of forming neurons) is largely finished after about 5 months of “gestation,” approximately the time at which prematurely born infants have some chance of surviving. During the next 5 months, until just after full-term birth, the fetal brain is especially delicate and extremely vulnerable to injury, “teratogens”, and “trauma.” (Kolb, 195) Also referred to as 'embryonic development' and ‘embryogenesis.’

Embryo: an animal at an early stage of development, before birth. (OxfordMed) The undeveloped organism during the period in which it is nourished only from stored food. Strictly speaking this term is applied to the young organism only while still enclosed in the egg membranes. (Richards, 395) A prenatal human is considered an embryo for the first 8 weeks. During this time rudiments of all body parts form. (Lewis, 53)

Embryonic Development Stages: the early stages of development in a multicellular organism during which the organization of the organism is largely formed. (Brooker, G-12) Editor's note - stage 1 is “fertilization."

Cleavage (stage 2): the cell divisions by which the egg is converted into a multicellular organism, the blastula. (Richards, 394) About a day after fertilization, the zygote divides by "mitosis," beginning a period of frequent cell divisions. (Lewis, 53) Succession of rapid cell divisions with no significant growth. (Brooker, G-7) The action of splitting or dividing. Cell division, especially of a fertilized egg cell. (Oxford) Also referred to as the ‘blastula stage of development.’

Embryonic Disc: a flat disc of "tissue" bounded “dorsally” by the “amniotic cavity” and “ventrally” by the “yolk sac.” (OxfordMed) During the second week of prenatal development, a space called the 'amniotic cavity' forms between the inner cell mass and the outer cells anchored to the uterine lining. Then the inner cell mass flattens into a two-layered embryonic disc. (Lewis, 55)

Gastrulation (stage 3): a process in which an area in the “blastula” invaginates and folds inward, creating different embryonic cell layers called “germ layers.” (Brooker, G-15) A process of complicated … cell movements that reorganizes a bilayer embryo into one with three “germ layers” and specific orientation. Gastrulation describes the germ layer development of a non-mammalian blastula or that of a mammalian blastocyst. (MeSH)

Archenteron: the primitive gut of the gastrula which later becomes the primitive "digestive" tract. (Richards, 392) Cylindrically shaped cavity (‘pouch’) formed during gastrulation that will become the organism’s digestive tract. (Booker, 1114)

Blastopore: a small opening created when a band of tissue invaginates during gastrulation. It forms the primary opening of the archenteron to the outside. (Brooker, G-5) The (connecting passage) between the ‘gastrular' cavity and the outside. (Richards, 393) 

Invagination: buckling inward of cells. (Booker, 1114) The unfolding process by which, for example in gastrulation, the primary “endoderm” is withdrawn into the blastula cavity and becomes enclosed by the primary “ectoderm.” (Richards, 398)

Primary Germ Layer: a primary formative layer of cells which by subsequent processes of development produces tissues and organs of the embryo. (Richards, 397) Embryonic cell layers (including) the ectoderm, the "mesoderm," and the endoderm. Will become "organs." (Brooker, G-16) Once these layers form, many cells become 'determined' to develop as a specific cell type. Each primary germ layer gives rise to certain structures. (Lewis, 55) Also referred to as 'germ layer.'

Ectoderm: the outer germ layer or the formative tissue composing it. (Richards, 395) Destined to become the ‘outer skin.’ Future structures include the "epidermis" (skin) and derivatives like "horns" and antennas. Includes the "nervous system," epidermal ("somatosensory receptors"), cornea and lens of the eye, tooth enamel, ear wax, sweat glands, and mammaries. (Norman, 7/8/09) Adjective - ‘ectodermal.’ Also referred to as ‘epiblast.’

Endoderm: the inner germ layer or the formative tissue composing it. (Richards, 396) Destined to become the ‘inner skin.’ Future structures include the epithelial linings of the digestive, reproductive, excretory, and respiratory systems. They include the liver, pancreas, stomach and other internal "glands" and organs. (Norman, 7/8/09) Adjective - ‘endodermal.’ Also referred to as ‘hypoblast.’

Mesoderm: the middle germ layer or the formative tissue composing it. (Richards, 399) Destined to become the ‘middle skin.’ Future structures include the "skeletal" system, muscular system, excretory system, reproductive system and most of the "circulatory" system. Also include the "notochord," and the lining of chest cavity. (Norman, 7/8/09)

Neurulation (stage 4): process whereby the embryo internalizes its developing nervous system. (Patestas, 12) The multistep embryological process responsible for initiating central nervous system formation. Occurs just after “gastrulation” and involves the formation of the neural tube from ectoderm located “dorsal” to the “notochord.” All neurons and their supporting cells in the central nervous system originate from neural 'precursor' cells derived from the neural tube. (Booker, 1116) By the twenty-eighth day, the neural tube seals up, completely isolating if from the outside world. (Bainbridge, 44)

Organogenesis (stage 5): the origin and development of the organs and organ systems in the embryo. (Richards, 400) The developmental stage during which cells and tissues form organs. (Booker, 1118) The process by which cells specialize and organize to form the tissues and organs of an organism. (SDBCoRe)

Embryonic Structures: a fertilized egg first becomes transformed into a cluster of cells without specialized functions. (Brooker, 1109) Every organism above the “protozoa” consists of many cells, and every one starts as a single-celled egg. The first problem of development for the fertilized egg, therefore, is that of the regular distribution of its substance in such a manner that the organs of the later stages may normally arise from the appropriate material. (Richards, 20)

Blastocyst: an early stage of embryonic development that consists of a hollow ball of cells with a localized thickening that will develop into the actual embryo. At first the blastocyst is unattached, but it soon implants in the wall of the uterus. (OxfordMed) The hollow sphere formed in mammals by the "cleavage" of the egg and the migration of the ”blastomeres.” (Richards, 393) During cleavage, the ball of cells hollows out, and its center fills with fluid, creating a blastocyst. (Lewis, 53) The mammalian counterpart of a 'blastula.' (Brooker, 1112)

Blastomere: the cell produced by cleavage of the egg. (Richards, 393) The two half-size daughter cells produced by each cell division during cleavage. The outer single-cell layer of blastomeres forms a sheet of epithelial cells that separates the embryo from its environment. (Brooker, 1110)

Gastrula: shortly after (the embryonic disc is formed), a third layer, the "mesoderm," forms in the middle. This three-layered structure is called the gastrula. (Lewis, 55) Embryo with primitive gut and 3 germ layers. (Norman, 7/8/09)

Morula: a type of blastula characterized by the absence of a (blastula) cavity. (Richards, 398) An early stage of embryonic development formed by cleavage of the fertilized “ovum.” It consists of a solid ball of cells and is an intermediate stage between the “zygote” and blastocyst. (OxfordMed)

Zygote: fertilized egg, or embryo after fertilization. (Richards, 405) Single cell formed at conception from the “chromosomes” of the biological mother and the biological father. (Hockenbury, 354) A "diploid cell" formed by the fusion of two gametes that (then) divides and develops into an embryo, and eventually into an adult organism. (Brooker, G-40) Also referred to as the ‘fertilized egg.’

Fetal Period: prenatal development after the 8th week, when structures grow and specialize. From the start of the ninth week until birth, the prenatal human organism is a "fetus." (Lewis, 51) The final and longest stage of prenatal development. The main task during the next seven months is for body systems to grow and reach maturity in preparation for life outside the mother’s body. By the fifth month, all the brain cells the person will have at birth are present. After birth, the communication links between these brain cells continue to develop in complexity. During the final two months, the fetus will double in weight, gaining an additional three to four pounds of body fat. As birth approaches, growth slows and the fetus’s body systems become more active. (Hockenbury, 356)

Fetus: the advanced embryo of the mammal. (Richards, 396) From the start of the ninth week until birth, the prenatal human organism is a fetus. (Lewis, 53) Also referred to as ‘foetus.’

Pregnancy: the period during which a woman carries a developing fetus, normally in the "uterus." Lasts for approximately 266 days, from conception until the baby is born. (OxfordMed) Nine month period (during which) a single cell develops into the estimated trillion cells that make up a newborn baby. (Hockenbury, 355)

Prenatal Diagnostic Tests: tests sometimes done during pregnancy that look for birth defects and "genetic" problems in the developing baby. (PubMedHealth) Screening tests identify fetuses that are at increased risk of having “trisomy 21” ("Down Syndrome"). If screening tests find a fetus to be at elevated risk, more invasive diagnostic tests are offered. (Lewis, 240)

Amniocentesis: prenatal diagnostic test. Examines fetal cells from "amniotic fluid." Detects large-scale chromosomal abnormalities. (Lewis, 238) A normal result means no genetic or chromosome problems were seen in the baby. An abnormal result may mean the baby has a gene or chromosome problem, such as Down syndrome and many others, or birth defects that involve the spine or brain, such as 'spina bifida.' (PubMedHealth) A sample of amniotic fluid is taken and fetal cells in it are examined for biochemical, genetic, and chromosomal anomalies. (Lewis, 56)  ‘Transabdominal’ puncture of the uterus during pregnancy to obtain amniotic fluid. It is commonly used for fetal "karyotype" determination in order to diagnose abnormal fetal conditions. (MeSH)

Cell-Free Fetal DNA, RNA: compares fetal "genome sequence" to that of the parents to reveal inherited diseases. Can be analyzed to detect "mutations." (Uses) fetal "DNA" that is free in the blood, rather than in cells. Pieces are much shorter than pieces of maternal DNA. Pieces cover the entire fetal genome. (Lewis, 392) A new technique detects fetal "mRNA" in mother’s blood. (Lewis, 241) Also referred to as ‘free fetal DNA.’

Chorionic Villi Sampling: a test done on some pregnant women to screen the baby for genetic problems. (PubMedHealth) A method for diagnosis of fetal diseases by sampling the cells of the placental “chorionic villi” for DNA analysis, presence of "bacteria," concentration of "metabolites," etc. The advantage over amniocentesis is that the procedure can be carried out in the first trimester. (MeSH) Examines chromosomes from cells snipped off the chorionic villi at 10 weeks. Because the villi cells and the embryo's cells come from the same fertilized ovum, an abnormal chromosome in villi cells should also be in the embryo. (Lewis, 55-56)

Chromosome Microarray Analysis (CMA): detects “copy number variants” (losses or gains of chromosomal material), including small sections of missing or extra DNA. (Lewis, 238) Methods used to detect copy number variations, which may be benign, "pathologic," or of unknown clinical significance. A far more sensitive method than traditional karyotyping, CMA detects both large and small copy number variations. Depending on the method used, CMA may involve scanning of the whole genome, or of a specific chromosome or a chromosome segment. The CMA methods used most commonly in clinical practice include ‘oligonucleotide array,’ ‘single nucleotide polymorphism array,’ and ‘oligo/SNP combination array.’ (GeneReviews)

Flow Cytometry: separates fetal cells from maternal blood by identifying surface characteristics that differ from those on the mother’s cells. The fetal cells are then karyotyped and specific gene tests performed on fetal DNA. (Lewis, 214) Technique using an instrument system for making, processing, and displaying one or more measurements on individual cells obtained from a cell suspension. Cells are usually stained with one or more "fluorescent dyes" specific to cell components of interest, e.g., DNA, and the "fluorescence" of each cell is measured as it rapidly transverses the a "laser" or 'mercury arc lamp beam.' Fluorescence provides a quantitative measure of various biochemical and biophysical properties of the cell, as well as a basis for cell sorting. (MeSH) Analysis of biological material by detection of the light-absorbing or fluorescing properties of cells or subcellular fractions (i.e., chromosomes) passing in a narrow stream through a laser beam. An absorbance or fluorescence profile of the sample is produced. Automated sorting devices sort successive droplets of the analyzed stream into different fractions depending on the fluorescence emitted by each droplet. (HGPIA) Also referred to as ‘fluorescence-activated cell sorter.’

Multiple Marker Screening: a blood test done during pregnancy to determine whether the baby is at risk for certain birth defects. (PubMedHealth) Detects abnormal levels of certain "proteins" in the pregnant woman’s blood. (Lewis, 140) Also referred to as ‘multiple maternal serum marker test’ and ‘quadruple screen test.’

Preconception Testing: checks couples for many "recessive" diseases before they have children. Test detects 448 recessive diseases that affect children. (Lewis, 392) Also referred to as 'preconception comprehensive carrier screening.’ 

Pregnancy Ultrasound: an imaging test that uses "sound waves" to see how a baby is developing in the "womb." It is also used to check the female pelvic organs during pregnancy. (PubMedHealth)

Prenatal Support Structures: as an embryo develops, structures form that support and protect it. (Lewis, 55)

Amniotic Sac: a fluid-filled protective housing for the embryo. (Hockenbury, 356) Toward the end of the embryonic period, the amniotic sac swells with fluid that cushions the embryo and maintains a constant "temperature" and "pressure." (Lewis, 56)

Amnion: the (inner) membrane that forms initially over the “dorsal” part of the embryo but soon expands to enclose it completely within the amniotic cavity. It is connected to the embryo at the “umbilical cord.” (OxfordMed)

Amniotic Cavity: the fluid-filled cavity between the embryo and the amnion. (OxfordMed)

Amniotic Fluid: the fluid contained within the amniotic cavity. It surrounds the growing (embryo and) fetus, protecting it from external pressure. (OxfordMed) Contains fetal urine and cells. (Lewis, 56)

Chorionic Villi: fingerlike outgrowths (that) extend from the area of the embryonic disc close to the "uterine" wall. The villi project into pools of the woman's blood. Her blood system and the embryo's are separate, but nutrients and "oxygen" diffuse across the chorionic villi from her "circulation" to the embryo. (Lewis, 55-56)

Placenta: an organ within the uterus by means of which the embryo is attached to the wall of the uterus. Its primary function is to provide the embryo with nourishment, eliminate its wastes, and exchange "respiratory" gases. It also functions as a "gland," "secreting" “hormones” which regulate the maintenance of pregnancy. (OxfordMed) A disk-shaped "vascular" organ that prevents the mother’s blood from directly mingling with that of the developing embryo. Acting as a filter, prevents many harmful substances that might be present in the mother’s blood from reaching the embryo. It cannot, however, filter out all harmful agents from the mother’s blood. (Hockenbury, 356)

Umbilical Cord: the strand of tissue connecting the fetus to the placenta. It contains two "arteries" that carry blood to the placenta and one "vein" that returns it to the fetus. (OxfordMed) The embryo’s lifeline that extends from the placenta on the mother’s uterine wall to the embryo’s abdominal area. Delivers nourishment, oxygen, and "water" and carries away "carbon dioxide" and other wastes. (Hockenbury, 356) Its cells are valuable. They can be cultured to differentiate as cells from any of the three primary germ layers, including "bone," "fat," "nerve," "cartilage," and muscle cells. "Stem cells" from the cord are used to treat a respiratory disease of newborns that scars and inflames the "lungs." They are abundant, easy to obtain and manipulate, and can become almost any cell type. (Lewis, 57)

Uterus: the part of the female reproductive tract that is specialized to allow the embryo to become implanted in its inner wall and to nourish the growing fetus from the maternal blood. (OxfordMed) (The fertilized ovum) settles into the lining of the uterus, where it may continue to divide and an embryo develops. If fertilization does not not occur, the "oocyte," along with much of the uterine lining, is shed (during "menstruation"). (Lewis, 46) Adjective - ‘uterine.’ Also referred to as 'womb.'

Yolk Sac: the membranous sac, composed of mesoderm lined with endoderm, that lies “ventral” to the embryo. Probably assists in transporting nutrients to the early embryo and is one of the first sites where blood cells are formed. (OxfordMed) Manufactures blood cells. (Lewis, 56)