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Embryonic Period
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The embryonic period is the first eight weeks of development of the conceptus, beginning with fertilization. During this period, the embryo undergoes an amazing transformation from a single cell to a recognizably human embryo. In the mid-twentieth century, George Streeter devised a commonly used embryo classification system by dividing the embryonic period into 23 stages. The embryonic period is followed by the fetal period, which encompasses the remainder of the prenatal development.
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Other Names
Related terms
- Embryogenesis
- Organogenesis
The first week: Pre-implantation
The first week of the embryonic period begins with the last phase of fertilization and ends with the attachment of the blastocyst to the endometrium. The preimplantation phase of the embryonic period is chiefly characterized by cell proliferation and early differentiation.
- Fertilization (Stage 1; Day 1), resulting in formation of zygote
- Cleavage (Stage 2; Days 2-3), resulting in formation of morula
- Blastocyst formation (Stage 3; Days 4-5), and differentiation of inner cell mass and trophoblast
- Blastocyst attachment (Stage 4; Day 6)
 Embryonic Stage 1: the last phase of fertilization in mouse. (1 & 2) Formation of ootid, which has two pronuclei in its center; (3 & 4) Formation of the one-cell embryo by dissolution of pronuclear membranes; and (5 & 6) Beginning of the first mitotic division of the zygote. Gray's Anatomy (1918) figure 8 (originally from Sobotta). Source: Wikimedia Commons |
 Cleavage. (a-d) Embryonic stage 2: Two-cell embryo, four-cell embryo, eight-cell embryo, and morula (16-cells); (e) early embryonic stage 3: Blastocyst (32-cells). Gray's Anatomy (1918) figure 9 (originally from Allen Thomson). Also shown: zona pellucida (z.p.) and polar bodies (p.gl.). Source: Wikimedia Commons |
 Blastocyst (Stage 4) in uterine lumen. Source: Wikimedia Commons |
The second week: Implantation of the Blastocyst (Stage 5)
During the second week, the conceptus becomes implanted within the endometrium. This second phase of the embryonic period, implantation, is also the "week of twos": the embryo is a bilaminar embryonic disk, the trophoblast consists of two distinct layers (cytotrophoblast and syncytiotrophoblast), and there are two extraembryonic vesicles (amniotic cavity, dorsal to the embryo, and yolk sac, ventral).
 Blastocyst of a bat (Vespertilio murinus) showing differentiation within the inner cell mass: epiblast ("inner cell mass") and hypoblast ("entoderm"). The blastocyst cavity is labelled "blastodermic vesicle." Gray's Anatomy (1918) figure 10 (originally from van Beneden). Source: Wikimedia Commons |
 Transverse section of bilaminar embryonic disc of a bat showing the epiblast ("ectoderm"), hypoblast ("entoderm"), amniotic ectoderm, cytotrophoblast and syncytiotrophoblast. Most of the trophoblast and blastocyst cavity are not shown. Gray's Anatomy (1918) figure 12 (originally from van Beneden). Source: Wikimedia Commons |
Weeks 3-8: Embryogenesis
The final part of the embryonic period begins with the completion of implantation. This phase, embryogenesis, is also referred to as organogenesis, or the critical or sensitive period because this is time when most of the major organ systems are formed, and the embryo is most vulnerable to teratogens. It also used to be common to call this time period the embryonic period, but that meaning is outdated.
Formation of the trilaminar embryonic disc (Stages 6 & 7; Week 3)
 Primitive streak (pr) on the dorsal surface of the bilaminar embryonic disc. Gray's Anatomy (1918) figure 13 (originally from Kölliker). Source: Wikimedia Commons |
 Dorsal view of gibbon (Hylobates concolor) embryo showing primitive streak and notochordal process. Gray's Anatomy (1918) figure 14 (originally from Selenka). Source: Wikimedia Commons |
 Conceptus in the third week (trilaminar embryonic disc). Chorion is composed of trophoblast (tr.) and extra-embryonic mesoderm (mes.). Note the presence of lacunae in the trophoblast, and secondary villi (villi with a mesoderm core, but no blood vessels). Gray's Anatomy (1918) figure 32 (originally from Peters). Source: Wikimedia Commons |
 Conceptus at about 3 weeks. Note formation of allantoic diverticulum and connecting (or body) stalk. Gray's Anatomy (1918) figure 25 Source: Wikimedia Commons |
Formation of the embryo proper (Stages 8 - 10; Week 4)
- Folding of the embryonic disk
- Neurulation
 Transverse section of a chick embryo at a comparable stage of development prior to embryo folding. Note differentiation of longitudinal bands in the mesoderm. Gray's Anatomy (1918) figure 19 (originally from Balfour). Source: Wikimedia Commons |
 Transverse section showing the lateral folding of the embryonic disc. Separate fusions of the splanchnopleure and somatopleure will later enclose the primitive gut and peritoneal cavity, respectively. Gray's Anatomy (1918) figure 29 (originally from Quain's Anatomy). Source: Wikimedia Commons |
 Dorsal view of Stage 10 human embryo (2.1 mm) with 8 somites. Gray's Anatomy (1918) figure 20 (originally from Eternod). Source: Wikimedia Commons |
 Lateral view of Stage 10 human embryo (2.6 mm). Gray's Anatomy (1918) figure 22 (originally from His). Source: Wikimedia Commons |
Growth and development of the embryo proper (Stages 11-23; Weeks 5-8)
 Transverse section of embryo during the 5th week showing differentiation of a somite into sclerotome (sc) and dermatomyotome (s.p. and m.p.). Gray's Anatomy (1918) figure 64 (originally from Kollmann). Source: Wikimedia Commons |
 Five-week old human embryo from ectopic pregnancy. Source: Wikimedia Commons. Photo by Ed Uthman, MD |
 Seven-week old human embryo from ectopic pregnancy. Source: Wikimedia Commons. Photo by Ed Uthman, MD |
Other Resources
O'Rahilly R and Müller F. Human Embryology & Teratology, 3rd Ed. New York: Wiley-Liss, 2001.
External Links
Human Developmental Anatomy Center (HDAC) of the National Museum of Health & Medicine
- O'Rahilly R & Müller F. Developmental Stages in Humans. Carnegie Institution of Washington, 1987.
Embryology.ch - an online course in embryology for medical students
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