Archive image from page 90 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 FROM LAYING TO FORMATION OF FIRST SOMITE 71 the posterior half or two fifths of the circular pellucid area (Fig. 35 B). It is relatively narrow in front and widens posteriorly, where it is at the same time less dense. Its anterior end usually does not quite reach the center of the pellucid area. It rapidly increases in length; the anterior end appears to be practically a fixed point, and growth takes place posteriorly probably not by addi
Archive image from page 90 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 FROM LAYING TO FORMATION OF FIRST SOMITE 71 the posterior half or two fifths of the circular pellucid area (Fig. 35 B). It is relatively narrow in front and widens posteriorly, where it is at the same time less dense. Its anterior end usually does not quite reach the center of the pellucid area. It rapidly increases in length; the anterior end appears to be practically a fixed point, and growth takes place posteriorly probably not by addition, but between the two ends. The posterior half of the pellucid area elongates simultaneously, keeping pace with the Fig. 35. — Surface views of two stages of the blastoderm of the egg of the sparrow. (After Schauinsland.) A. Before the appearance of the primitive streak. B. The first appearance of the primitive streak. a. o., Area opaca. a. p., Area pellucida. Ent. Th., Thickening of en- toderm, pr. str., Primitive streak. primitive streak which lies entirely within it in the chick and most other birds. Thus the area pellucida becomes oval, then pear-shaped, and the primitive streak bisects the greater part of its length (Figs. 35, 36, 44, etc.). According to Keller the primitive streak takes its origin from a crescentic area at the posterior margin of the pellucid area, which he terms the sickle. The primitive streak appears as a process extending forward from the center of the sickle, and, as it grows forward, the lateral horns of the sickle are gradually taken into its posterior end. Roller's observations and interpretations have not, however, been con- firmed by subsequent investigators and they would appear to rest on rather exceptional and inessential conditions.
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