. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. FORCE WHICH CAUSES THE BLASTOMERES TO ADHERE 327 derm itself two types of cells may be observed, namely, a superficial ecto- dermal cell and a more deeply situated, somewhat vacuolated, smaller ento- dermal cell (fig. 175B). (For later stages of blastulation, see chapter 7.) e. Cleavage in the California Hagfish, Polistotrema (Bdellostoma) stouti The California hagfish spawns an egg which is strongly telolecithal. The germinal disc (blastodisc) is
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. FORCE WHICH CAUSES THE BLASTOMERES TO ADHERE 327 derm itself two types of cells may be observed, namely, a superficial ecto- dermal cell and a more deeply situated, somewhat vacuolated, smaller ento- dermal cell (fig. 175B). (For later stages of blastulation, see chapter 7.) e. Cleavage in the California Hagfish, Polistotrema (Bdellostoma) stouti The California hagfish spawns an egg which is strongly telolecithal. The germinal disc (blastodisc) is situated immediately below the egg membrane at one end of the egg, adjacent to the micropyle and the anchor filaments (fig. 162A). Cleavage begins in this disc, and the enlarging blastoderm slowly creeps downward to envelop the massive yolk material. The freshly laid egg measures about 29 mm. by 14 mm., including the shell. Without the shell, the egg is about 22 mm. by 10 mm. and is rounded at each end (Dean, 1899). The first two cleavage planes may be regarded as meridional (or vertical) (fig. 162B). The third cleavage appears to be a mixture of vertical and hori- zontal (latitudinal) cleavages, with the former predominating (fig. 162D, E). Cleavage from this time on becomes irregular, and a typical meroblastic blasto- derm soon is attained with central and marginal cells (fig. 162F). C. What is the Force Which Causes the Blastomeres to Adhere Together During Early Cleavage? A question naturally arises concerning the force which makes the blasto- meres of most chordates adhere to one another during the early cleavage ANCHOR FILAMENT MIC ROP YL BLA STODE (G ER MIN A PLASM) BLASTODERM. Fig. 162. Egg and cleavage in the marine lamprey, Polistotrema (Bdellostoma) stouti. After Dean, 1899.) (A) Animal pole end of the egg. (B) Surface view of blasto- dermic hillock, showing first cleavage furrow. (C) Same, second cleavage. (D) Third cleavages. (E, F) Later cleavages, strongly irreg
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