. The science and art of midwifery. layer, and termed the embryonic spot,or by some authors the protosoma, becauseit represents the most primitive stage inthe development of the embryo. The ovalshape of the embryonic spot is suggestiveof the future distinction between the headand the posterior extremity, the largerend corresponding to the former, and thesmaller to the latter. Then, of a sudden,there appears in the middle of the embry-onic spot a delicate line termed the prim-itive trace, which divides it into two later-al halves. The primitive trace consists ofa groove or furrow, bordered by t
. The science and art of midwifery. layer, and termed the embryonic spot,or by some authors the protosoma, becauseit represents the most primitive stage inthe development of the embryo. The ovalshape of the embryonic spot is suggestiveof the future distinction between the headand the posterior extremity, the largerend corresponding to the former, and thesmaller to the latter. Then, of a sudden,there appears in the middle of the embry-onic spot a delicate line termed the prim-itive trace, which divides it into two later-al halves. The primitive trace consists ofa groove or furrow, bordered by two ridges,termed the dorsal plates, and formed bya thickening of the external layer. The dorsal plates may be readilyunderstood by reference to the transverse section (Fig. 39), takenfrom Professor Daltons Treatise on Human Physiology. Upon microscopic examination of such a transverse section, theembryo is found to be composed of three layers, which, in the verte-brata, are united together in the median line. The intermediate layer. Fig. 39.—Transverse section ofegcr in early stage of develop-ment. 1, external and medianlayers of blastodermic mem-brane ; 2, 2, dorsal plates; 3,internal layer ot blastodermicmembrane. (Dalton.)
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Keywords: ., bookcentury1800, bookdecade1890, bookidsci, booksubjectobstetrics
