The elements of embryology (1874) The elements of embryology elementsofembryo01fost Year: 1874 III.] THE NOTOCHOED. 49 Fig. ii. me Surface View of the Pellucid Area of a Blastoderm of 18 hours. None of the opaque area is shewn, the pear-shaped outline indicating the limits of the pellucid area. At the hinder part of the area is seen the primitive groove pr., with its nearly parallel walls, fading awav behind, but curving round and meeting in front so as to form a distinct anterior termination to the groove, about half way up the pellucid area. Above the primitive groove is seen the medulla
The elements of embryology (1874) The elements of embryology elementsofembryo01fost Year: 1874 III.] THE NOTOCHOED. 49 Fig. ii. me Surface View of the Pellucid Area of a Blastoderm of 18 hours. None of the opaque area is shewn, the pear-shaped outline indicating the limits of the pellucid area. At the hinder part of the area is seen the primitive groove pr., with its nearly parallel walls, fading awav behind, but curving round and meeting in front so as to form a distinct anterior termination to the groove, about half way up the pellucid area. Above the primitive groove is seen the medullary groove m. c, with the medullary folds A. These diverging behind, slope away on either side of the primitive groove, while in front they curve round and meet each other close upon a curved line which represents the head-fold. The second curved line in front of and concentric with the first is the com- mencing fold of the amnion. groove, however, the mesoblast is thinned out and very soon the cells in this position, separating from the lateral masses, adhere together in the middle line, and thus form between the epiblast and the hypoblast a flattened circular rod known as the notochord, seen in section as an elliptical aggregation of cells (Fig. 12, ch.) e. 4
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