Carnegie stages

In embryology, Carnegie stages are a standardized system of 23 stages used to provide a unified developmental chronology of the vertebrate embryo.

The stages are delineated through the development of structures, not by size or the number of days of development, and so the chronology can vary between species, and to a certain extent between embryos. It only covers the first 60 days of development; at that point the term embryo is usually replaced with the term fetus.

It was based on work by Streeter (1942) and O'Rahilly and Müller (1987). The name "Carnegie stages" comes from the Carnegie Institute.

While the Carnegie stages provide a universal system for staging and comparing the embryonic development of most vertebrates, other systems are occasionally used for the common model organisms in developmental biology, such as the Hamburger-Hamilton stages in the chick.

Stages
Days are approximate, and reflect the days since the last ovulation before pregnancy ("Postovulatory age").

Stage 1: 1 days

 * fertilization
 * polar bodies

Stage 2: 3 days

 * cleavage
 * morula
 * compaction

Stage 3: 4 days

 * blastocyst and blastocoele
 * embryonic disk
 * trophoblast and embryoblast

Stage 4: 5-6 days

 * syncytiotrophoblast
 * cytotrophoblast
 * amniotic ectoderm

Stage 5 (a-c): 7-12 days

 * implantation
 * bilaminar germ disk
 * primary yolk sac
 * amniotic cavity

Stage 6: 13-15 days

 * primitive streak
 * primitive groove
 * chorionic villi
 * secondary yolk sac

Stage 7: 15-17 days

 * gastrulation
 * neural plate
 * start of hematopoiesis
 * notochord

Stage 8: 17-19 days

 * primitive pit

Stage 9: 19-21 days

 * neural groove
 * neural folds
 * septum transversum
 * placode
 * early heart

Stage 10: 21-23 days

 * pharyngeal arches #1 and #2
 * cardiac loop
 * intermediate mesoderm

Stage 11: 23-26 days

 * sinus venosus
 * mesonephric duct

Stage 12: 26-30 days

 * upper limb buds

Stage 13: 28-32 days

 * septum primum
 * foramen primum

Stage 14: 31-35 days

 * ureteric bud

Stage 18: 44-48 days

 * septum secundum