. Comparative anatomy. Anatomy, Comparative. 74 COMPARATIVE ANATOMY intact during this process of segmentation of the lateral folds, is the mate- rial of the future notochord (Fig. 46, NC). At the time of hatching, then, the embryo has made important progress beyond the gastrula stage. Not only has the segregation of mesoderm begun but two important organs, the central nerve tube and the notochord, are indicated. After hatching, additional pairs of mesodermal segments are cut off from the lateral mesodermal folds, the addition taking place progressively from anterior to posterior, until a tota
. Comparative anatomy. Anatomy, Comparative. 74 COMPARATIVE ANATOMY intact during this process of segmentation of the lateral folds, is the mate- rial of the future notochord (Fig. 46, NC). At the time of hatching, then, the embryo has made important progress beyond the gastrula stage. Not only has the segregation of mesoderm begun but two important organs, the central nerve tube and the notochord, are indicated. After hatching, additional pairs of mesodermal segments are cut off from the lateral mesodermal folds, the addition taking place progressively from anterior to posterior, until a total of ordinarily fourteen pairs have been produced. In several of the more posterior segments cavities do not occur, the mesodermal folds merely breaking up into a succession of soHd blocks of ^MES 1-6^ e'c Fig. 47.—AMPHIOXUS. Frontal (horizontal) section of an embryo having six pairs of mesodermal somites. The section is through the notochord and just below the blastopore. At the posterior end of the section may be seen a region where the notochord, endoderm and mesoderm merge indistinguishably. A, archenteron near the blastopore; EC, ectoderm; EN, endoderm; MES 1-6, mesodermal somites; NC, notochord. (After Cerfontaine.) In the formation of these fourteen pairs of mesodermal pouches the material of the original mesodermal folds is completely utilized. These folds, however, terminated posteriorly in the close vicinity of the blasto- pore (Fig. 47) a region where, since an early gastrula stage, very rapid cell proliferation has been going on. Following, then, the formation of the fourteen pairs of mesodermal segments from the lateral folds, the series of segments is extended backward by addition of successive soHd blocks of cells which become detached from the growth zone encirchng the blasto- pore. By this means the number of pairs of mesodermal units, the earlier and more anterior developing as hollow endodermal outgrowths, the later and more posterior as solid block
Size: 2184px × 1144px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookpublisherphi, booksubjectanatomycomparative
