. Text-book of embryology. Embryology. 162 INVEETEBEATA CHAP. give rise by budding to strings of cells which will form the ventral nerve-cord. Outside these are two cells termed " nephridioblasts," which give rise to strings of cells which separate into groups as the body of the embryo leech lengthens, and form the nephridia. Outside these again are the myoblasts, which similarly give rise to cells which form the external circular muscles, whilst finally the two most external cells give rise to strings of cells which broaden oiit and form the posterior ectoderm. All these strings of
. Text-book of embryology. Embryology. 162 INVEETEBEATA CHAP. give rise by budding to strings of cells which will form the ventral nerve-cord. Outside these are two cells termed " nephridioblasts," which give rise to strings of cells which separate into groups as the body of the embryo leech lengthens, and form the nephridia. Outside these again are the myoblasts, which similarly give rise to cells which form the external circular muscles, whilst finally the two most external cells give rise to strings of cells which broaden oiit and form the posterior ectoderm. All these strings of cells taken collectively constitute the trunk blastema, it will be thus seen that the development of the trunk blastema is almost identical in character in Grio- drilus and Nephelis. A few of the nephridia which are formed at first grow to relatively enormous dimensions, and function as excretory organs during development (Sukatschoff, 1900). These larval nephridia later disappear, and one is involuntarily reminded of the fate which befalls pronephros of Vertebrata (Figs. 121 and 122). At first the head blastema is widely separated from this trvmk blastema by a large expanse of bare macromeres: in a typical annelidan embryo this expanse would be taken up by the prototrochal. Fig. 121.—Hinder view of a well de- veloped larva of Nephelis vulgaris. (After Sukatschoff.) Letters as before. In addition, , larval 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 MacBride, E. W. (Ernest William), 1866-1940; Kerr, John Graham, 1869-; Heape, Walter, 1855-1929. London : Macmillan
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