. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 324 CLEAVAGE (SEGMENTATION) AND BLASTULATION. Fig. 160. Cleaving eggs of Platypus and Echidna. (After Flynn and Hill, '39.) (A) Egg, shell, and early cleavage in Ornithorhynchus. (B) Early cleavage in Echidna. See fig. 161D. synchronization of mitotic division is lost. In certain other teleost fishes, lati- tudinal cleavages begin as early as the 8-cell stage. At the 32- to 64-cell stages in Serranus atrarius, the blastoderm presents a cap-like ma
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. 324 CLEAVAGE (SEGMENTATION) AND BLASTULATION. Fig. 160. Cleaving eggs of Platypus and Echidna. (After Flynn and Hill, '39.) (A) Egg, shell, and early cleavage in Ornithorhynchus. (B) Early cleavage in Echidna. See fig. 161D. synchronization of mitotic division is lost. In certain other teleost fishes, lati- tudinal cleavages begin as early as the 8-cell stage. At the 32- to 64-cell stages in Serranus atrarius, the blastoderm presents a cap-like mass of dividing cells overlying a forming blastocoel (fig. 159H, I). Between the blastocoel and the yolk, there is a thin layer of protoplasm connecting the edges of the cap. This thin protoplasmic layer is the forerunner of the central periblast tissue; at this stage it contains no nuclei (fig. 159F, H). 2) Origin of the Periblast Tissue in Teleost Fishes. In the sea bass and many other teleost fishes, some of the surrounding cells at the edge of the blastoderm lose their cell boundaries and fuse together to form a common syncytial tissue. The nuclei in this tissue continue to divide (fig. 159J) and eventually migrate into the periblast tissue below the blastocoel (see arrow, fig. 159L). The latter then becomes the central periblast, while the syncytial tissue around the edges of the growing blastodisc forms the peripheral or marginal periblast (fig. 159K-M). In the trout, the early cleavage furrows of the blastodisc are incomplete, and the periblast arises from the syncytial tissue established directly below and at the sides of the protoplasmic cap (fig. 159N-R). This condition re- sembles the cleavage process in the elasmobranch fishes. See Kerr ('19); Kopsch ('11); and H. V. Wilson (1889). d. Prototherian Mammalia The Prototheria normally are placed in the class Mammalia along with the Metatheria (marsupials) and Eutheria (true placental mammals). How-. Please note that thes
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