. The elements of Embryology,. III.] THE HYPOBLAST. 51 We must now go back, and say a few words about the changes which the cells of the various layers undergo from the 12—20 hours. Fig. Tkansvekse Section op a Blastoderm incubated for rS hours. Tlie section passes through the medullary groove mc, at some distance behind its extreme front, and shews some of the chief points in which it differs from the piimitive groove. The chief of these are, (i) the presence underneath it of the notorhord ch., (2) the absence of any apparent adhesion between the and the meso- blast, (3) the tl
. The elements of Embryology,. III.] THE HYPOBLAST. 51 We must now go back, and say a few words about the changes which the cells of the various layers undergo from the 12—20 hours. Fig. Tkansvekse Section op a Blastoderm incubated for rS hours. Tlie section passes through the medullary groove mc, at some distance behind its extreme front, and shews some of the chief points in which it differs from the piimitive groove. The chief of these are, (i) the presence underneath it of the notorhord ch., (2) the absence of any apparent adhesion between the and the meso- blast, (3) the tliickeriing of the mesoblast underneath the medullary folds, mf. A. epiblast. B. mesoblast. C. hypoblast, m. c. medullary groove, medullary fold. c/;. notochord : the small group of mesoblast cells separated by a narrow gap from the thicker mass of mesoblast on either side. It is to be noticed that the cells of the hypoblast become more columnar as they approach the edge of the pellucid area, and finally pass, without any strong line of demarcation, into the white-yolk spheres. Only one half of the section is represented—if completed the section would be symmetrical about the line passing through the centre of the medullary canal, mc. The hypoblast (Fig. 12, C) continues to be only one cell deep; the cells being, during the whole of this period, flatter in the centre, and larger and more irregular towards the peri- phery of the blastoderm. At about the 12th hour they are very irregular in size; shewing very great variations over a very small space. This probably implies that they are rapidly undergoing division. Later, however (about the 18th hour), they are fairly uniform over particular regions, though they vary considerably in size at different parts of the pellucid area. In no case does the hypoblast extend beyond the edge of the pellucid area. The hypoblast cells along the central axis of the pellucid area, and for some little distance on each side, are smaller tha
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