ABSTRACT
The 23 Carnegie stages of human embryos will be used as the reference base in this atlas and will be presented in two ways:
The presomite stages (Carnegie stages 1 to 8) are listed (Table 3) but not illustrated because: (a) teratological research is rarely, if every carried out during this period; (b) staging requires microscopic examination; and (c) staging is largely dependent upon details of implantation which are not comparable with those of other species. Carnegie Stages 1 to 8 of Human Embryos
Carnegie stage
Age (days)
Features
1
1
Fertilization
2
2—3
From 2 to about 16 cells
3
4—5
Free blastocyst
4
5—6
Attaching blastocyst
5
7—12
Implanted though previllous
6
13—15
Chorionic villi; primitive streak may appear
7
15—17
Notochordal process
8
17—19
Primitive pit; notochordal and neurenteric canals
Stages 9 to 23 cover the all-important period of embryogenesis. These stages will be illustrated and a brief account of the internal features diagnostic of each stage will be given. The criteria used to identify a particular stage are: (a) greatest length, (b) known or estimated age, and (c) external features. In general, the greatest length and external features will be the most reliable criteria.
(A) Left lateral and dorsal views of a reconstruction of Carnegie embryo No. 1878. (B) Left lateral view of the same embryo showing ectodermal areas of head region (stippled) and the mesencephalic flexure. https://s3-euw1-ap-pe-df-pch-content-public-p.s3.eu-west-1.amazonaws.com/9781351069939/5bbd806d-0715-46b3-b0f7-7252d28d5864/content/fig_4.jpg"/> (From O’Rahilly, R., Developmental stages in human embryos, including a survey of the Carnegie collections. Part A. Embryos of the first three weeks (Stages 1 to 9), Carnegie Institute of Washington, D.C., 1973. With permission.)