. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 360 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE. TROPHOBLA S T CELLS EPIDERMAL ECTODERM NEURAL ECTODERM NOTO CHORD PRE-CHORDAL PLATE BLASTOCOEL HYPOBLAST Fig. 173. Presumptive organ-forming areas in the chick blastoderm. (A) Slightly modified from Spratt, '46. (B) Schematic section of early chick blastoderm passing through antero-posterior median axis. tion from the midline of the future embryonic axis. Anterior to the neural crescent is the presum
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 360 THE CHORDATE BLASTULA AND ITS SIGNIFICANCE. TROPHOBLA S T CELLS EPIDERMAL ECTODERM NEURAL ECTODERM NOTO CHORD PRE-CHORDAL PLATE BLASTOCOEL HYPOBLAST Fig. 173. Presumptive organ-forming areas in the chick blastoderm. (A) Slightly modified from Spratt, '46. (B) Schematic section of early chick blastoderm passing through antero-posterior median axis. tion from the midline of the future embryonic axis. Anterior to the neural crescent is the presumptive epidermal crescent. Within the area opaca is found potential blood-vessel and blood-cell-forming tissue, as well as the extensive extra-embryonic-tissue materials. The above description of the presumptive organ-forming areas pertains to the avian blastula just previous to the inward migrations of the notochordal, pre-chordal plate, and mesodermal areas; that is, just previous to the ap- pearance of the primitive streak and the gastrulative process. 4. Primary and Secondary Reptilian Blastulae The primary blastula of turtle, snake, and lizard embryos is akin in essen- tial features to that of birds. It consists of a central blastoderm or area pellucida, overlying a primary blastocoelic cavity, and a more distally situ- ated opaque blastoderm, together with an indefinite periblast syncytium. A localized region of the central blastoderm, situated along the midline of the future embryonic axis and eccentrically placed toward the caudal end, is known as the embryonic shield. A specialized, posterior portion of the embryonic shield, in which the upper layer (epiblasi) is not separated from the underlying cells (hypoblast), is known as the primitive plate (fig. 174A-D). (Consult also Will, 1892, for. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not
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