. The development of the chick; an introduction to embryology . op. Ves. ■ Hi. j 0;;:-^ Ao. -3. IS. ■■^ -pr str. Fig. 63. —Embryo of 12 s, from above, drawnas a transparent object with transmittedlight. X 30. Abbreviations as before. IV. The Mesoblast The changes in the mesoblast during this period are of greatimportance. At the time of appearance of the liead-fold it con-sists of two great sheets of cells between ectoderm and entoderm no THE DEVELOPMENT OF THE CHICK beginning on each side of the head-process and primitive streak,and extending laterally and posteriorly to the
. The development of the chick; an introduction to embryology . op. Ves. ■ Hi. j 0;;:-^ Ao. -3. IS. ■■^ -pr str. Fig. 63. —Embryo of 12 s, from above, drawnas a transparent object with transmittedlight. X 30. Abbreviations as before. IV. The Mesoblast The changes in the mesoblast during this period are of greatimportance. At the time of appearance of the liead-fold it con-sists of two great sheets of cells between ectoderm and entoderm no THE DEVELOPMENT OF THE CHICK beginning on each side of the head-process and primitive streak,and extending laterally and posteriorly to the margin of thevascular area. The lateral margins at this time extend anterior tothe embryonic axis, so that the anterior margin of the mesoblastforms a curve with the concavity directed Fig. 64. — Head of the same embryo frombelow. X 30. Abbreviations as before. The mesoblast in the region in front of the primitive streakis known as gastral mesoblast, and in the region of the primitivestreak as prostomial mesoblast; the latter is fused with the primi-tive streak. However, the distinction between the gastral andprostomial mesoblast is not of permanent significance, becausethe latter is being continually converted into the former as theprimitive streak undergoes separation into ectoderm, notochord,and mesoderm. Confining our account now to the gastral mesoblast: a trans-verse section across an embryo in which the head-fold is formingshows a sheet of cells lying on each side of the notochord betweenthe ectoderm and entoderm. It is several cells deep near thenotochord, and thins gradually peripheralh (cf. Fig. 56). Thethicker portion next the notochord is distinguished as the paraxialmesoblast (vertebral plate) from the more peripheral portion orlateral plate. The meso
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