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. In the human being only the first 60 days of development are covered; 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 Institution of Washington. 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 Carnegie stage 1 is the unicellular embryo. This stage is divided into three substages. Stage 1 a Primordial embryo. All the genetic material necessary for a new individual, along with some redundant chromosomes, are present within a single plasmalemma. Penetration of the fertilising sperm allows the oocyte to resume meiosis and the polar body is extruded. Stage 1 b Pronuclear embryo. Two separate haploid components are present - the maternal and paternal pronuclei. The pronuclei move towards each other and eventually compress their envelopes where they lie adjacent near the centre of the wall. Stage 1 c Syngamic embryo. The last phase of fertilisation. The pronuclear envelopes disappear and the parental chromosomes come together in a process called syngamy. Stage 2: 2-3 days cleavage morula compaction Carnegie stage 2 begins when the zygote undergoes its first cell division, and ends when the blastocyst forms a cavity and comprises more than 16 cells. At this point, it is called a morula. The cleavage divisions of CS2 embryos do not occur synchronously. And the fate of the blastomeres is not yet determined. The two-cell embryo is spherical and surrounded by the transparent zona pellucida. Each of the blastomeres that form is also spherical. On approximately day 3, at the eight-cell stage, compaction usually begins. Stage 3: 4-5 days blastocyst and blastocoele trophoblast and embryoblast Carnegie stage 3 begins when a cavity first appears in