. The development of the chick; an introduction to embryology . Fig. 5L — Embryo of 4 s from above, drawn in alcohol by reflected light. a. c. v., Amnio-cardiac vesicle, a. p., Area pellucida. a. v. i., Inter-nal vitelline area. med. pi., Medullary plate, n. F., Neural fold. Pra.,Proamnion, pr. str., Primitive streak, s. 1, s. 3, First and thirdsomites. 98 THE DEVELOPMENT OF THE CHICK anterior end of the plate (Fig. 48) (the neural groove); the mar-gins of the anterior portion of the medullary plate then becomeelevated somewhat above the surrounding blastoderm, forming. eip • ji --■>-♦ f %
. The development of the chick; an introduction to embryology . Fig. 5L — Embryo of 4 s from above, drawn in alcohol by reflected light. a. c. v., Amnio-cardiac vesicle, a. p., Area pellucida. a. v. i., Inter-nal vitelline area. med. pi., Medullary plate, n. F., Neural fold. Pra.,Proamnion, pr. str., Primitive streak, s. 1, s. 3, First and thirdsomites. 98 THE DEVELOPMENT OF THE CHICK anterior end of the plate (Fig. 48) (the neural groove); the mar-gins of the anterior portion of the medullary plate then becomeelevated somewhat above the surrounding blastoderm, forming. eip • ji --■>-♦ f % * Fig. 52. — The same embryo from beneath,a. c. v., Amnio-cardiac vesicle, a. i. p., Anterior intestinal F., Head-fold. Pra., Proamnion. the neural folds (Figs. 51 and 56). The latter rise very rapidly,,thus deepening the neural groove, and bend in towards the middleline (Figs. 53,54, etc.,) meeting,by the time four or five somites are HEAD-FOLD TO TWELVE SOMITES 99 formed, a short distance back of the anterior end of the medullaryplate (Figs. 50 and 51). The posterior ends of the neural foldsdo not, at this time, reach the region of the first somite. Theregion where the neural folds first come in contact correspondsapproximately with the region of the future mid-brain, or ante-rior part of the hind-brain.
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