. Text-book of embryology. Embryology. VII ANNELIDA 161 may say at once that these " vegetative micromeres," as we may term them give rise to the posterior ectoderm and the external circular muscles. They form in fact part of the trunk blastema. The endodermal epithelium does not arise as in other Annelida trom the fourth and fifth quartettes of micromeres and from the equal division of the residual macromeres, but by the budding of a single cell from the residual macromere 2B, and of several cells from the residual macromere 4D. The residual macromeres in the quadrants A, B, and C,
. Text-book of embryology. Embryology. VII ANNELIDA 161 may say at once that these " vegetative micromeres," as we may term them give rise to the posterior ectoderm and the external circular muscles. They form in fact part of the trunk blastema. The endodermal epithelium does not arise as in other Annelida trom the fourth and fifth quartettes of micromeres and from the equal division of the residual macromeres, but by the budding of a single cell from the residual macromere 2B, and of several cells from the residual macromere 4D. The residual macromeres in the quadrants A, B, and C, after having undergone the divisions recorded above, give rise to no more cells; they become smaller and smaller as development goes on, and are finally absorbed. The cells destined to form the lining of the alimentary canal lie between them, and the remains of the macromeres are thus found outside the alimentary canal; a contrast to the condition of affairs obtaining in the Ehynco- bdellidae as represented by Clepsine, where the endoder- mal cells are budded from the surfaces of all four macro- meres and surround them. Thus, in one group of leeches the endoderm is formed from practically only one macromere, and lies in- side surrounded by the four macromeres, whilst in another group of leeches, the Ehyn- chobdellidae, including Clepsine (Glossiphonia) and its allies (Fig. 120), the endoderm is formed from all four macromeres and lies outside them. We conclude that both forms of development are modifica- tions of a primitive type, such as is seen in Polygordius and most Polychaeta, in which the residual macromeres are directly converted into the endodermal epithelium; and we are reminded of somewhat similar differences in endoderm formation between Siphonophora and other Hydrozoa, amongst Coelenterata (Chap. IV), and between Planocera and Tungia amongst Platyhelminths (Chap. V). In JVephelis, after the endoderm is formed, a transverse row of ten cells can be discerned at the hi
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