The Carnegie Stages are the 23 internationally standardized stages of human embryonic development — from fertilization (CS 1, day 1) to the end of the embryonic period (CS 23, day 56). The convention goes back to George L. Streeter (1942—1951, Carnegie Institution of Washington), was revised by Ronan O’Rahilly and Fabiola Müller, and today serves as the standard reference (O’Rahilly & Müller, Cells Tissues Organs 192:73—84, 2010).
The stages are ordered not by days in the womb but by morphologically defined steps. They are thus the only internationally comparable point of reference for embryonic development — independent of individual variation in the speed of development.
Significance for personal ontology
The Carnegie Stages describe how the human person unfolds in the first dimension — they do not describe whether and when it comes to be. In terms of personal ontology, a person is present from CS 1 onward (cf. embryo, fertilization, beginning of human existence). The Carnegie convention supplies the morphological vocabulary by which the claims of personal ontology can be empirically anchored — it does not replace them.
Three reference points are especially relevant to the argument:
- CS 1 (day 1) — zygote after syngamy. Beginning of the new organism with totipotency. The personal-ontological beginning.
- CS 6 (day 17) — appearance of the primitive streak, beginning of gastrulation. End of the possibility of monozygotic twinning. Classical boundary of the ISSCR’s 14-day rule. Pivot point of the debate over individuation (Smith/Brogaard 2003 vs. Damschen/Schönecker 2006 vs. Condic 2020).
- CS 23 (day 56) — end of the embryonic period. All organ primordia laid down. Transition to the fetal phase (week 9).
Complementary to the morphological Carnegie convention, the HuDeCA Cell Atlas has since 2019 supplied cellular resolution: which cell types exist at which stage, into which lineage trajectories they diverge, at which spatial positions they appear. Where Carnegie describes the what kind of form, HuDeCA describes the which cells within it.
Table of the 23 stages
| CS | Day(s) post fert. | Defining event | Description |
|---|---|---|---|
| 1 | Day 1 | Zygote | Fertilization, single-celled zygote after syngamy. |
| 2 | Day 2—3 | Cleavage, morula | First cleavage divisions up to the morula (2—16 cells). |
| 3 | Day 4—5 | Free blastocyst | Free blastocyst with blastocoel; ICM/trophoblast differentiation; hatching from the zona pellucida. |
| 4 | Day 6 | Attaching blastocyst | Apposition of the blastocyst to the endometrium. |
| 5 | Day 7—12 | Implantation, bilaminar germ disc | Implantation, lacunar trophoblast, formation of the bilaminar germ disc. |
| 6 | Day 17 | Primitive streak, onset of gastrulation | Appearance of the primitive streak; onset of gastrulation; classical boundary of the 14-day rule. |
| 7 | Day 19 | Notochordal process | Formation of the notochordal process; gastrulation advances. |
| 8 | Day 23 | Primitive pit, neural plate | Primitive pit; neural plate appears. |
| 9 | Day 25 | Neural folds, first somites | Neural folds; first 1—3 somites; cardiac primordium. |
| 10 | Day 28 | Neural tube fusion, pharyngeal arches 1—2 | Onset of neural tube fusion; pharyngeal arches 1 and 2; first heartbeat. |
| 11 | Day 29 | Rostral neuropore, optic vesicle | Closure of the rostral neuropore; optic vesicle. |
| 12 | Day 30 | Caudal neuropore, upper limb bud | Closure of the caudal neuropore; upper limb buds; arches 3—4. |
| 13 | Day 32 | Lower limb bud | Lower limb buds; lens placode; otocyst. |
| 14 | Day 33 | Lens pit, ureteric bud | Lens pit; ureteric bud. |
| 15 | Day 36 | Lens vesicle, hand plates | Lens vesicle closed; hand plates. |
| 16 | Day 39 | Foot plates, retinal pigment | Foot plates; retinal pigment visible. |
| 17 | Day 41 | Finger rays | Finger rays; nasolacrimal groove. |
| 18 | Day 44 | Toe rays, eyelid primordium | Toe rays; eyelid primordium; onset of ossification. |
| 19 | Day 46 | Trunk elongation | Elongation of the trunk. |
| 20 | Day 49 | Elbow flexion | Upper limbs bent at the elbow. |
| 21 | Day 51 | Free fingers | Hands and feet reach the midline; fingers free. |
| 22 | Day 53 | Eyelids, external ear | Eyelids and external ear well developed. |
| 23 | Day 56 | End of embryonic period | End of the embryonic period; all organ primordia laid down; transition to the fetal phase (week 9). |
Pluripotency hierarchy along the stages
The ISSCR Standards for Human Stem Cell Use in Research (2023) codify the cells’ scope of differentiation along the stages:
- CS 1—2 (day 1—3) — totipotency: zygote and early blastomeres can form both embryonic and extra-embryonic tissue.
- CS 3 (day 4—5) — naive pluripotency: inner cell mass (ICM) of the free blastocyst.
- CS 5 (day 7—12) — primed pluripotency: post-implantation epiblast.
- From CS 10 onward — gradual specification into multi- and unipotent lineages (e.g., hematopoietic stem cells, spermatogonia).
The pluripotency hierarchy is a statement about cells, not about the personhood of the organism to which they belong.
Fields of application
- Bioethics of reproductive medicine (preimplantation genetic diagnosis, embryo transfer, synthetic embryo models) — most ART interventions take place between CS 1 and CS 5.
- Research ethics — the ISSCR’s 14-day rule effectively sets the boundary at CS 6 (primitive streak).
- Law and regulation — many jurisdictions date legal or clinical thresholds to Carnegie Stages (implantation, primitive streak).
- Personal ontology — the stages supply the morphological vocabulary by which the argument of personal ontology is empirically anchored.
Sources: Generated by querying the Personhood ontology.
Further sources:
- O’Rahilly, R. & Müller, F. (2010): Developmental Stages in Human Embryos: Revised and New Measurements. Cells Tissues Organs 192(2): 73—84. (Standard reference of the Carnegie convention.)
- Streeter, G. L. (1942—1951): Developmental Horizons in Human Embryos. Carnegie Institution of Washington, Contributions to Embryology, vols. 30—34. (Classical first version.)
- Sadler, T. W. (2023): Langman’s Medical Embryology. 15th ed. Wolters Kluwer.
- Carlson, B. M. (2018): Human Embryology and Developmental Biology. 6th ed. Elsevier.
- ISSCR (2023): Standards for Human Stem Cell Use in Research. International Society for Stem Cell Research.
- Endowment for Human Development: Developmental Stages in Human Embryos (Carnegie Collection) — https://www.ehd.org/developmental-stages/stage0.php.
- Mark Hill (UNSW): Embryology — Carnegie Stage Comparison — https://embryology.med.unsw.edu.au/embryology/index.php/Carnegie_Stage_Comparison.
See also
- Embryo — personal-ontological classification of the human embryo
- Fertilization — the initial event (CS 1)
- Beginning of Human Existence — six steps to the question
- Individuality — the question of individuation (CS 6)
- Synthetic Embryo Model — SCBEMs as models of natural stages
- Preimplantation Genetic Diagnosis — selection between CS 3 and CS 4
- Surplus Embryo — the downstream problem of IVF