. The comparative anatomy of the domesticated animals. Horses; Veterinary anatomy. GENERAL DIRECTION OF DEVELOPMENT. 101$ The tergale zone is occupied by a new formation, which appears to be the continuation of the primitive trace, but is in reality altof^ether distinct. This ia a furrow formed by a longitudinal excavation in the ectoderm, and which com- mences the trace of the central nervous system; it is named the medullar// groove, or dorsal furrow. At the bottom of this groove a dark line betrays the presence of the chorda dorsalis, or notochord—a provisional skeletal stalk that sustains
. The comparative anatomy of the domesticated animals. Horses; Veterinary anatomy. GENERAL DIRECTION OF DEVELOPMENT. 101$ The tergale zone is occupied by a new formation, which appears to be the continuation of the primitive trace, but is in reality altof^ether distinct. This ia a furrow formed by a longitudinal excavation in the ectoderm, and which com- mences the trace of the central nervous system; it is named the medullar// groove, or dorsal furrow. At the bottom of this groove a dark line betrays the presence of the chorda dorsalis, or notochord—a provisional skeletal stalk that sustains the medullary groove. The presence of the medullary groove leads to the division of the embryo into two perfectly distinct zones—the spinal zone, corresponding with the- medullary groove, and the marginal zme comprising all the other part to the borders of the embryo. These facts become very intelligible in a transverse section through the middle of the medullary groove (Fig, 551). It is seen how the latter is formed by the thickening and median inflection of the ectoderm^ Fig. en TRAKSVERSE SECTION OF A BLASTODERM AFTER THE FORMATION OF THK GROOVE AND THE NOTOCHORD. A, Ectoderm ; B, mesoderm ; C, endoderm mc, medullary groove ; mf, medullary fold ; ch, noto- chord. The figure represents the left half of the section. which resembles a V widely open, the two branches of the Y constituting the medullary folds (Fig. 550). By their inflection and union they ultimately form a closed tube—the neural canal, perhaps better designated as the neuraxis (Fig. 552). The endoderm does not offer any other modification, but the mesoderm undergoes transformations of the highest interest. The interposition of the chorda dorsalis divides it into two symmetrical moieties. The thickened parts, which, in the two moieties, are subjacent to the corresponding medullary fold, form the latc'ral layers (or plates). Beyond, the mesoderm shows, in certain embryos at this time, a very
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