A system of obstetrics . Cross-section of Embryo of chick througl) the Primitive Streak, the blastoderm being of aboutthe same age as that represented in Pig. 21 (from Balfour). The section passes acimiddle of the primitive streak : , primitive streak ; ep, epiblast; hy, hypoblast. THE EARLY STAGES OF DEVELOPMENT. 107 coincides with the long axis of the future embryo. The embryo, how-ever, develops entirely in front of the primitive streak, the front end Fig. Fig. 24. Vertical Transverse Section through Primitive Streak of Developing Hens Egg, after the lateraloutgrowths of the mesobl
A system of obstetrics . Cross-section of Embryo of chick througl) the Primitive Streak, the blastoderm being of aboutthe same age as that represented in Pig. 21 (from Balfour). The section passes acimiddle of the primitive streak : , primitive streak ; ep, epiblast; hy, hypoblast. THE EARLY STAGES OF DEVELOPMENT. 107 coincides with the long axis of the future embryo. The embryo, how-ever, develops entirely in front of the primitive streak, the front end Fig. Fig. 24. Vertical Transverse Section through Primitive Streak of Developing Hens Egg, after the lateraloutgrowths of the mesoblast have commenced : pr, primitive streak and commencing prim-itive groove; m, mesoblast; ep, epiblast; hy, hypoblast (from Balfour. Most of the right halfof the section is omitted, the middle line of the blastoderm being at pr). of which corresponds to its tail end. Along its axis the mesoblast ofthe primitive streak remains for some time continuous with the epiblast:but its cells multiply and spread laterally(Fig. 23, m), and there form a separatelayer. After about sixteen hours of in-cubation the area pellucida is seen to haveincreased considerably in size and to havebecome pear-shaped. Along its axis ap-pears a superficial furrow, the primitivegroove (pr, Fig. 24). The primitive groove and streak aretransient structures in the chick, and areonly explicable as due to heredity fromancestral forms. Comparative embryologyhas, in fact, afforded a satisfactory e
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectobstetrics, bookyear1
