. Comparative anatomy. Anatomy, Comparative. REPRODUCTION 91 If "anus" is to be defined as the posterior aperture of the digestive tube—digestive in strict sense—then the term should not be applied to the external opening of the cloaca but rather to the internal aperture between rectum and cloaca. By this definition the proctodeal aperture would ordinarily become the external cloacal opening but in some cases would become an external anus (Teleostei) or an internal anus (frog). If the posterior extremity of the adult mammalian digestive tube includes a remnant of the embryonic cloaca
. Comparative anatomy. Anatomy, Comparative. REPRODUCTION 91 If "anus" is to be defined as the posterior aperture of the digestive tube—digestive in strict sense—then the term should not be applied to the external opening of the cloaca but rather to the internal aperture between rectum and cloaca. By this definition the proctodeal aperture would ordinarily become the external cloacal opening but in some cases would become an external anus (Teleostei) or an internal anus (frog). If the posterior extremity of the adult mammalian digestive tube includes a remnant of the embryonic cloaca, then the mammalian "anus" is in reality a remnant of an external cloacal aperture. There is much con- fusion and doubt about homologies in this region of the vertebrate and, among authors, there is corresponding confusion of terms. The Mesoderm. In contrast to the early condition of the mesoderm in Amphioxus, the vertebrate mesoderm is at first devoid of segmentation. Fig. 62.—Transverse sections of embryos of the amphibian Ambystoma; A, earlier stage; B, later and more posterior than A. a, archenteron; e, otic pit; en, endoderm; m, mesoderm; n, notochord; nc, neural crest; ng, neural groove; np, neural plate; p, primitive groove. (From Kingsley, Comparative Anatomy of Vertebrates.) and ordinarily contains no definite cavity. At an early stage it appears as a somewhat compactly arranged but indefinitely delimited sheet of cells rapidly spreading from its region of origin into the space between ectoderm and endoderm. The layer becomes thickest dorsally alongside the neural tube and notochord (Fig. 62). At an early embryonic stage the mesoderm upon either side spHts into two layers; an outer, lying against the ectoderm, and an inner lying against the endoderm. The two layers remain connected, however, at the upper edge of the original sheet (Figs. 63, 64A). At about the same time the dorsal and thicker part of the mesoderm develops transverse fissures which divi
Size: 2402px × 1040px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookpublisherphi, booksubjectanatomycomparative
