. The comparative anatomy of the domesticated animals. Veterinary anatomy. EAKLY EMBRYONIC DEVELOPMENTS 1011 close together, separated by a bright line (primitive streak), but continuous with each other in front. All the transparent zone in front of the primitive line has been designated by Duval as the tergal zone, and we adopt this correct and convenient term. The growth of the primitive line takes place at its posterior extremity, and to this circumstance is due the change in shape of the transparent area, which becomes pyriform. It is indispensable to see, on transverse sections, the arran
. The comparative anatomy of the domesticated animals. Veterinary anatomy. EAKLY EMBRYONIC DEVELOPMENTS 1011 close together, separated by a bright line (primitive streak), but continuous with each other in front. All the transparent zone in front of the primitive line has been designated by Duval as the tergal zone, and we adopt this correct and convenient term. The growth of the primitive line takes place at its posterior extremity, and to this circumstance is due the change in shape of the transparent area, which becomes pyriform. It is indispensable to see, on transverse sections, the arrangement of the blastoderm and its layers at the primitive trace. The ectoderm, formed of a layer of cubical cells, is curved in the middle, and its deep face becomes con- founded with the mesoderm. The latter, formed of small angular cells, is placed between the two layers, and enters the substance of the opaque zone. The endoderm, formed of a single layer of fiat cells throughout the extent of the trans- parent zone, suddenly becomes thicker at the opaque zone, by the stratification of its cells still filled with vitelline granules. These marginal thickenings have been named by Kolliker the endodermic ridges; it is to them that the opaque zone owes its optical characters. This transverse section enables us to see, for the first time, the three super- Fig. TRANSVERSE SECTION OF A BLASTODERM OF THE SAME AGE AS FIG. 547. pos., Primitive trace; ep., epiblast, or ectoderm; hy., endoderm ; yk., above, the endodermic ridge. The section passes nearly through the middle of the primitive trace. posed layers of the germinal disc ; and it exhibits, between the ectoderm and the mesoderm, a curious and characteristic continuity of the primitive trace, which has for a long time exercised the wisdom of embryologists, and given rise to debates now terminated. We will say nothing of the solution arrived at, as it would lead us too far from the scope of this work ; and only refer to the pri
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