. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 7° THE MEDULLARY GROOVE indication of the medullary groove, whose walls are destined to give rise to the central nervous system. Sections passing through the region of the medullary groove show, lying beneath it, the head process (Fig. 42, hp), already fused with the endoderm (compare p. 57), and on each side of the process is a plate of mesoderm (gm), representing the gastral mesoderm of lower forms (see Figs. 28 and 34), but not as yet showing any indications of splitting into the two layer
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 7° THE MEDULLARY GROOVE indication of the medullary groove, whose walls are destined to give rise to the central nervous system. Sections passing through the region of the medullary groove show, lying beneath it, the head process (Fig. 42, hp), already fused with the endoderm (compare p. 57), and on each side of the process is a plate of mesoderm (gm), representing the gastral mesoderm of lower forms (see Figs. 28 and 34), but not as yet showing any indications of splitting into the two layers that bound the embryonic ccelom (see p. 59). am. Fig. 42.—Section through the Frassi Embryo just in Front of the Neuren- teric Canal. am, Amniotic cavity; gm, gastral mesoderm; hp, head process; mp, medullary plate; ys> yolk-sac.—(Frassi.) This is just beginning to appear in an embryo, also described by von Spee and known as embryo Gle. It measured mm. in length and is closely similar, in general appearance, to an embryo described by Eternod and measuring mm. in length (Fig. 43). It differs from the Frassi embryo most markedly in that the posterior portion of the embryonic disk, that is to say the primitive streak region, is bent ventrally to lie almost at a right angle with the anterior portion. As a result the belly-stalk arises from the ventral surface of the embryo instead of from its posterior extremity, near which the opening of the neurenteric canal (Fig. 43, nc) is now situ- ated, almost the whole length of the surface seen in dorsal view being occupied by the medullary groove (m), which, in the embryo Gle, is bounded laterally by distinct ridges, the medullary Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original McMurrich, J. Playfair (James Playfair), 1859-1939. Philade
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