The cell in development and inheritance . cover othercharacters that more or less clearly foreshadow the later develop-ment. One of the most interesting and clearly marked of these isthe bilateral symmetry of the ovum in bilateral animals, which issometimes so clearly marked that the exact position of the embryomay be predicted in the unfertilized tgg, sometimes even before it islaid. This is the case, for example, in the cephalopod e^gg, as shownby Watase (Fig. 179). Here the form of the new-laid &gg, beforecleavage begins, distinctly foreshadows that of the embryonic body,and forms as it wer
The cell in development and inheritance . cover othercharacters that more or less clearly foreshadow the later develop-ment. One of the most interesting and clearly marked of these isthe bilateral symmetry of the ovum in bilateral animals, which issometimes so clearly marked that the exact position of the embryomay be predicted in the unfertilized tgg, sometimes even before it islaid. This is the case, for example, in the cephalopod e^gg, as shownby Watase (Fig. 179). Here the form of the new-laid &gg, beforecleavage begins, distinctly foreshadows that of the embryonic body,and forms as it were a mould in which the whole development is general shape is that of a hens egg shghtly flattened on one side, PROMORPHOLOGICAL RELATIONS OF CLEAVAGE 383 the narrow end, according to Watase, representing the dorsal aspect,the broad end the ventral aspect, the flattened side the posteriorregion, and the more convex side the anterior region. All the earlycleavage-furrows are bilaterally arranged with respect to the plane of a^. Fig. 180. —Eggs of the insect Corixa. [Metschnikoff.]A. Early stage before formation of the embryo, from one side. B. The same viewed in theplane of symmetry. C. The embryo in its final position. a. anterior end; p. posterior; /. left side, r. right; t/. ventral, d. dorsal aspect. (These lettersrefer to ih& final position of the embryo, which is nearly diametrically opposite to that in which itfirst develops) ; m. micropyle; near/ is the pedicle by which the egg is attached. symmetry in the undivided egg; and the same is true of the laterdevelopment of all the bilateral parts. Scarcely less striking is the case of the insect , as has beenpointed out especially by Hallez, Blochmann, and Wheeler (, 180). In a large number of cases the (tg^ is elongated and 384 CELL-DIVISION AND DEVELOPMENT bilaterally symmetrical, and, according to Blochmann and Wheeler,may even show a bilateral distribution of the yolk correspondingwith the bilateralit
Size: 1529px × 1634px
Photo credit: © The Reading Room / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1900, booksubjectcells, bookyear1902
