. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. CLEAVAGE AND DIFFERENTIATION. 27. homogeneous cells; such divisions are extremely constant and in many cases are visibly differential. Even in the case of the echinoderm Qgg it has been shown that four micromeres are constantly formed at one pole of the Qgg, and in this respect, at least, the cleavage here is determinate, for although Driesch has shown that a nor- mal larva develops from a sea- urchin tgg from which the mi- cromeres have been removed, this no more indicates, as Morgan ^ assumes,
. Biological lectures delivered at the Marine Biological Laboratory of Wood's Holl [sic]. Biology. CLEAVAGE AND DIFFERENTIATION. 27. homogeneous cells; such divisions are extremely constant and in many cases are visibly differential. Even in the case of the echinoderm Qgg it has been shown that four micromeres are constantly formed at one pole of the Qgg, and in this respect, at least, the cleavage here is determinate, for although Driesch has shown that a nor- mal larva develops from a sea- urchin tgg from which the mi- cromeres have been removed, this no more indicates, as Morgan ^ assumes, that these micromeres are undifferenti- ated and that the cleavage is, therefore, indeterminate than the fact that a hydra is able to complete itself and form a nor- mal hydra after its tentacles have been removed indicates differentiated. The one most striking feature of determinate cleavage is the constancy with which certain blastomeres give rise to certain organs, the invariable segregation of an entire region, layer, or organ into a single cell or particular group of cells. In all the gasteropods mentioned above the ectoderm comes from three quartettes of cells, each of which occupies relatively the same position and gives rise to the same organs (Fig. 4). The mesoderm comes from the posterior cell of the fourth quar- tette. All the other cells are entodermal, and, although they show certain variations in size and position in different genera, owing to variations in the amount and distribution of yolk, they are always constant for the same species. The four apical cells give rise to an apical sense organ (see Figs. 3-10), the trochoblasts and tip cells of the cross form the first velar row, the anterior arm of the cross forms the anterior cell plate, 1 Morgan, T. H., " A Study of a Variation in Cleavages," Arch, fiir Entwick- lungsmechanik, Bd. 2. Hft. i. Fig. 7. — Crepidula, log-cell stage (ninety-two ectoblast cells). Shading and heavy lines as in preceding f
Size: 1615px × 1546px
Photo credit: © Library Book Collection / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1890, bookpublishe, booksubjectbiology
