. Text-book of embryology. Embryology. Fig. 80.—Developing egg of Planocem inquilina. Eight - cell stage viewed from animal pole. (After Surface.) The 8-cell stage is succeeded by a 16-cell stage. The micro- meres divide into upper and lower cells by laeotropic spindles, and by four laeotropic spindles four new micromeres are budded off which alternate with the lower daughter cells of the first micro- meres. (Fig. 81.) The 16-cell stage is succeeded by a 32-cell stage. By dexio- tropic spindles the macromeres bud off a third set of micromeres. The second micromeres divide into upper and lower
. Text-book of embryology. Embryology. Fig. 80.—Developing egg of Planocem inquilina. Eight - cell stage viewed from animal pole. (After Surface.) The 8-cell stage is succeeded by a 16-cell stage. The micro- meres divide into upper and lower cells by laeotropic spindles, and by four laeotropic spindles four new micromeres are budded off which alternate with the lower daughter cells of the first micro- meres. (Fig. 81.) The 16-cell stage is succeeded by a 32-cell stage. By dexio- tropic spindles the macromeres bud off a third set of micromeres. The second micromeres divide into upper and lower cells, whilst each daughter of the first set of micromeres divides into an upper and lower cell. The subsequent history of the embryo proves that in Planocera, as in every Annelid and Mollusc that has been examined, the three groups of micromeres and their daughters constitute the entire ectoderm, whilst what is left of the macromeres, after the separa- tion of these micromeres, gives rise to the endoderm and mesoderm. Now if the four macro- meres were precisely equal in size it would be of course im- possible to distinguish them from one another, but in Planocera, as in the vast majority of cases, one is slightly larger than the rest and distinguishes itself by peouharities in its develop- ment after the micromeres have been given off; this macromere is denominated D. In all cases where it is recog- nizable from the first it forms a landmark by means of which the cleavage planes of the egg can be. correlated with the planes of symmetry of the adult. It is thus found that D is situated on what will be the posterior side of the embryo in the middle line. The other cells are named A, B, and C, following each other round a circle from left to right, in the same direction as the hands of a clock move when viewed from above. A and C are situated on the left and right sides respectively. Fig. 81.—Developing egg ot Planocera ingiuUina. Sixteen-cell stage viewed from tie animal pol
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