Archive image from page 93 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 74 THE DEVELOPMENT OF THE CHICK The later history of the primitive streak is ilhistrated in Figs. 44, 51, 61, 65, etc.: the embryo arises in front of it around the head-process as a center; the anterior end of the primitive streak marks the hind end of the differentiated portion of the embryo. As the embryo grows in length the primitive streak decreases (cf. measurements in table), until finally, when the completion of the embryo is indic
Archive image from page 93 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 74 THE DEVELOPMENT OF THE CHICK The later history of the primitive streak is ilhistrated in Figs. 44, 51, 61, 65, etc.: the embryo arises in front of it around the head-process as a center; the anterior end of the primitive streak marks the hind end of the differentiated portion of the embryo. As the embryo grows in length the primitive streak decreases (cf. measurements in table), until finally, when the completion of the embryo is indicated by the formation of the tail-fold, the primi- tive streak disappears. The primitive knot and primitive pit occupy its anterior end at all stages, and, as the embryo differen- tiates from the anterior end of the primitive streak, the primitive pit must be regarded as moving back along the line of the primi- tive groove, always representing its anterior end. Sections. The preceding sketch of the superficial appearance of the primitive streak must now be followed by a careful exami- nation of its structure and role in the development. - â m â '-WSBSiii-- Fig. 37. â Three sections through the primitive streak of a sparrow at a stage intermediate between Figs. 35 and 36. x 230. (After Schauinsland.) A. In front of the primitive streak. B. Through the anterior end of the primitive streak (primitive knot). C. About through the center of the primitive streak. All recent authors are agreed that the primitive streak owes its origin to a linear thickening of the ectoderm, from which cells are proliferated between the ectoderm and the entoderm, forming a third layer, the mesoderm. Figs. 37 A, B, C show three trans- verse sections through a blastoderm of the sparrow slightly more advanced than the stage shown in Fig. 35 B. The first section is just in front of the primitive streak. The ectoderm is thick in the center and thins gradually toward the margin of the area pellucida, be
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