. Amphioxus and the ancestry of the vertebrates [microform]. Vertebrates; Chordata; Fishes; Ascidiacea; Vertébrés; Cordés; Poissons; Ascidiacés. by the continued deepening of the archenteric cavity (Fig. 60). It is now a well-estabhshed fact that all multicellular animals (Metazoa) pass through a gastrula-stage in the course of their development, although the form of the gastrula is often extremely modified and difficult to recog- nise. The significance of this fact, as was long since pointed out bv Huxlev, Haeckel, Lan- kester, and others, is very great when it is remembered that the embryoni
. Amphioxus and the ancestry of the vertebrates [microform]. Vertebrates; Chordata; Fishes; Ascidiacea; Vertébrés; Cordés; Poissons; Ascidiacés. by the continued deepening of the archenteric cavity (Fig. 60). It is now a well-estabhshed fact that all multicellular animals (Metazoa) pass through a gastrula-stage in the course of their development, although the form of the gastrula is often extremely modified and difficult to recog- nise. The significance of this fact, as was long since pointed out bv Huxlev, Haeckel, Lan- kester, and others, is very great when it is remembered that the embryonic character- istics of the gastrula are essentially identical with the permanent features of the organisation of the Coelen- terata (Hydra, etc.). Returning to the gastrula begins to rotate witiun tiic foiiicic. ,, , , . ^ . , (.After IlATSCIlKK.) ot Amphioxus, \\\ the course of the further differentiation which goes hand in hand with the progressive growth antl development, we shall find that the primitive ectoderm gives rise to (i) the central nervous system and (3) the definitive ectoderm ; the primitive endoderm gives rise to (i) the viesoderni, \\\\\q\\ is usually regarded as a third or intermediate germ-layer ; (2) the notochord; and (3) the definitive endoderm, which forms the lining mucous epithelium of the alimentary canal ; finally, the primitive gastral cavity or archenteron will become subdivided into (1) the body-cavity or civ/oin, and (2) the definitive gut or alimentary canal. The embryo shown in optical section in Fig. 60 repre- sents the stage reached at midnight of the first night of development. It will be noticed that one side is convex, Fig. 60. — Optical lonfjiiiKlinal sec- tion of kiter gustruki. Cili;i (Hms^uILi) have been iMotmied from the ectocierni cells, and the enil)ryo at tliis stage. A. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of the
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectfishes, bookyear1894
