. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 176 Biology of the Vertebrates From these few brief statements concerning the early stages in the de- velopment of various vertebrates it is clear that gastrulation and the forma- tion of mesoderm and notochord vary considerably among the several verte- brate classes. It should be emphasized, however, that once the three primary germ layers become established they give rise to the principal organ systems with great uniformity, as shown in Figure 118. VI. T
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 176 Biology of the Vertebrates From these few brief statements concerning the early stages in the de- velopment of various vertebrates it is clear that gastrulation and the forma- tion of mesoderm and notochord vary considerably among the several verte- brate classes. It should be emphasized, however, that once the three primary germ layers become established they give rise to the principal organ systems with great uniformity, as shown in Figure 118. VI. THE MAJOR CAVITIES A coelomic cavity usually develops on each side of the body in the manner we have described, by the splitting of the mesodermal sheet. Hence it is a schizocoele (schizo, split). In amphioxus, however, the anteriormost mesoderm arises as a series of outpocketings, or pouches, from the dorso- Neural Groove Mesoderma Pouch. ^Mesodermal Pouch ^rr-1 ^Archenteron Segmentation Cavity Fig. 133. Formation of mesodermal pouches in amphioxus. (After Wilder.) lateral regions of the archenteron (Fig. 133). After the mesodermal pouches separate from the archenteron the cavities of all those on each side of the body combine to form a continuous coelom, usually known as an entero- coele because of its origin from the archenteron. This method of mesoderm formation does not occur in the vertebrates, with the probable exception of some amphibia, but is typical of hemichords and echinoderms. This forma- tion of enterocoeles in echinoderms and some chordates as well as the pos- terior position of the blastopore in echinoderms and all chordates indicates that these two phyla may be closely related. Mesenteries and other parts of the mesoderm arise in much the same manner whatever the method of formation of their germ layer may be. In. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearanc
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