. Principles of modern biology. Biology. Embryonic Development; Differentiation of Tissues - 281 remnants of the gill slits persist in the adult body (p. 283). Despite the large number of species, the Vertebrata are classified as a subphylum in the phylum Chordata (p. 664). In addition to the vertebrates, the phylum Chordata in- cludes a number of less familiar animals, which possess gill clefts and a dorsal hollow nerve cord, but no true vertebral column. In place of a vertebral column, primitive chor- dates possess an unsegmented flexible rod- like supporting structure, the notochord, which
. Principles of modern biology. Biology. Embryonic Development; Differentiation of Tissues - 281 remnants of the gill slits persist in the adult body (p. 283). Despite the large number of species, the Vertebrata are classified as a subphylum in the phylum Chordata (p. 664). In addition to the vertebrates, the phylum Chordata in- cludes a number of less familiar animals, which possess gill clefts and a dorsal hollow nerve cord, but no true vertebral column. In place of a vertebral column, primitive chor- dates possess an unsegmented flexible rod- like supporting structure, the notochord, which occupies an equivalent position in the body (Fig. 15-10). Vertebrate animals develop neural groove (Fig. 15-11, A,B), which runs lengthwise of the embryo and is broader at the anterior end (Fig. 15-8D). The neural groove is bounded laterally by the neural folds, which rise slightly from the surface of the embryo. Posteriorly the neural folds en- circle the blastopore (Fig. 15-8D), which is called the yolk plug because it is blocked by yolk cells. As development proceeds, the neural groove sinks below the surface of the embryo, and the neural folds come together along the mid-dorsal line (Fig. 15-11B,C). This in- vagination forms the neural tube, which will spinal cord, body wall digestive tract. BRAIN. GONADS ANUS Fig. 15-10. Amphioxus, a primitive chordate, sectioned longitudinally. Note especially: 1, the notochord; 2, the dorsal nervous system (brain and spinal cord; and 3, the gill slits. This animal cannot be classed as a vertebrate since it lacks a segmented vertebral column. a notochord, but only transiently, during the embryonic period, prior to the appearance of the segmented vertebral column. In fact, it is the possession of a notochord, as well as of gill clefts and a dorsal nervous system, that confirms the relationship between the vertebrates and more primitive chordate forms, and justifies placing all these animals in the same phylum. Origin of the Nerve Cord and
Size: 2836px × 881px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodiversity, booksubjectbiology
