Table 1.

Modern and other older anatomical terms of the early chick embryo.

modern terminology	description	comments	other older names	selected references
epiblast	uppermost cell layer		ectophyll, ectoblast, ectoderm	Lillie [2] and Romanoff [1]
hypoblast	initial extraembryonic lower layer under area pellucida (initially separate islands fuse to form a continuous sheet of cells)	extraembryonic fate: germinal crescent, yolk sac stalk	endophyll (‘primary hypoblast’; Vakaet, Stern 1990), sickle endoblast (‘secondary hypoblast’; Callebaut)	Vakaet [11], Eyal-Giladi & Kochav [17], Bellairs et al. [18], Stern [19] and Callebaut & van Nueten [20]
endoblast	posterior GWM-derived extraembryonic lower layer; displaces hypoblast sheet	extraembryonic, yolk sac endoderm	entoblast (Vakaet), junctional endoblast (Vakaet, Stern)	Vakaet [11], Callebaut & van Nueten [20], Stern [19] and Bachvarova et al. [21]
germ wall	lower layer of area opaca	peripheral extraembryonic endoderm (large yolky cells)	area opaca endoderm or area opaca endoblast (Vakaet, Stern)	Stern [19] and Bachvarova et al. [21]
germ wall margin	germ wall portion underlying the marginal zone (often protrudes to cover Koller's sickle)	peripheral extraembryonic endoderm (large yolky cells)—posterior portion generates endoblast	deep portion of marginal zone (Stern)	Stern [19] and Bachvarova et al. [21]
marginal zone (epiblast only)	intermediate ring of epiblast between area opaca and area pellucida; inner boundary is Koller's sickle	extraembryonic epiblast—no contribution to embryonic tissues	epiblast of the marginal zone (Eyal-Giladi), a superficial portion of the marginal zone (Stern)	Spratt & Haas [10], Eyal-Giladi & Kochav [17] and Bachvarova et al. [21]
definitive endoderm	embryonic endoderm; derived from tip of the primitive streak	fate: gut lining, other endodermal organs	entoderm, definitive endoblast, gut endoderm, endoderm	Bellairs [22], Stern & Ireland [23] and Kimura et al. [24]
Koller's sickle	crescent-shaped ridge located posteriorly; marks boundary between posterior area pellucida and marginal zone	contributes to primitive streak, some endoderm and mesoderm	Rauber's sickle (Callebaut)	Koller [25], Callebaut & van Nueten [20] and Bachvarova et al. [21]
mesoderm		fate: cardiovascular, musculoskeletal, etc. systems	mesoblast	Lillie [2] and Romanoff [1]