TABLE 1.

Definitions in early chick development

Primitive streak: The first three-dimensional structure of the avian embryo, whose orientation indicates the anterior-posterior axis. The appearance of this structure defines gastrulation, as cells of the epiblast ingress through the streak to generate the mesoderm and endoderm layers.

Hensen’s node: The cellular region at the anterior end of the primitive streak in avian embryos, which controls the development of the body plan during gastrulation. The homologous structures in mammals and amphibians are referred to as the Node and Organizer, respectively.

Rostral: The anterior end of the embryo (i.e., towards the head).

Caudal: The posterior end of the embryo (i.e., towards the tail).

Lateral mesoderm: Regions of the mesodermal layer that are bilaterally situated on both sides of the embryonic anterior-posterior midline. At the onset of gastrulation, this term is used to refer generally to the mesoderm that flanks the primitive streak. However, the mesoderm rapidly organizes into morphologically distinct regions, and therefore the definition of lateral mesoderm becomes more specific. At the time just prior to somite formation, the mesoderm that flanks the midline becomes subdivided into paraxial, intermediate, and lateral regions with the latter region giving rise to the heart.

Splanchnic mesoderm: The ventral layer of the lateral mesoderm, which lays adjacent to the endoderm. This tissue layer forms when lateral mesoderm separates into distinct dorsal and ventral layers: the somatic and splanchnic mesoderm. It is the splanchnic mesoderm that will give rise to the heart.

Hamburger and Hamilton (HH) staging: The commonly accepted system for staging avian embryogenesis, based on the sequence of morphological changes that occur during development.

HH stage 2: Beginning of gastrulation as the primitive streak begins to form at the posterior end.

HH stage 3: Early gastrulation with the primitive streak extending to the center of the embryo.

HH stage 4: The primitive streak is now extended to its most anterior position. Increased numbers of ingressing cells through the primitive streak leads to the formation of definitive mesoderm and endoderm layers.

HH stage 5: The anterior apex of the primitive streak has begun to recede in a posterior direction. The notochord is now visible as a rod of mesoderm extending above Hensen’s node.

HH stage 6: The anterior end of the embryo has folded to form the head fold. Lateral mesoderm begins to separate into dorsal and ventral layers.

HH stage 7: One somite stage. Splanchnic and somatic mesoderm are now distinct layers. First expression of sarcomeric myosin in the cardiogenic mesoderm.

HH stage 8: Four somite stage.

HH stage 9: Seven somite stage.

HH stage 10: Ten somite stage. The first display of beating by the primitive heart.