Archive image from page 129 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 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 the vascular area. The lateral margins at this time extend anterior to the embryonic axis, so that the anterior margin of the mesoblast forms a curve with the concavity directed forward. Fig. 64. — Head of the same embryo from below. X 30. Abbreviations as before. The mesoblast in th
Archive image from page 129 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 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 the vascular area. The lateral margins at this time extend anterior to the embryonic axis, so that the anterior margin of the mesoblast forms a curve with the concavity directed forward. Fig. 64. — Head of the same embryo from below. X 30. Abbreviations as before. The mesoblast in the region in front of the primitive streak is known as gastral mesoblast, and in the region of the primitive streak as prostomial mesoblast; the latter is fused with the pi-imi- tive streak. However, the distinction between the gastral and prostomial mesoblast is not of permanent significance, because the latter is being continually converted into the former as the primitive 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 forming shows a sheet of cells lying on each side of the notochord between the ectoderm and entoderm. It is several cells deep near the notochord, and thins gradually peripherally (cf. Fig. 56). The thicker portion next the notochord is distinguished as the paraxial mesoblast (vertebral plate) from the more peripheral portion or lateral plate. The mesoblast is sparser, the cells more scattered,
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