Archive image from page 93 of The development of the chick;. The development of the chick; an introduction to embryology . developmentofchi00lill Year: 1908 YiG. 37. — 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 pro
Archive image from page 93 of The development of the chick;. The development of the chick; an introduction to embryology . developmentofchi00lill Year: 1908 YiG. 37. — 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, becoming decidedly thin in the region of the area opaca. The thin entoderm of the area pellucida unites peripherally with the thick yolk-sac entoderm of the area opaca. The second
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