. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. TYPES OF CHORDATE BLASTULAE 359 separating the superficial cells from the deeper ones" (fig. 171B-D). He further suggests that "the bilaminar embryo of birds is to be homologized with the blastula of the Amphibia, the cleft separating the two layers being equivalent to the blastocoele" (p. 13). The formation of the hypoblast (pri- mary entoderm) by a process of delamination from the upper layer or epiblast agrees with the observatio
. Comparative embryology of the vertebrates; with 2057 drawings and photos. grouped as 380 illus. Vertebrates -- Embryology; Comparative embryology. TYPES OF CHORDATE BLASTULAE 359 separating the superficial cells from the deeper ones" (fig. 171B-D). He further suggests that "the bilaminar embryo of birds is to be homologized with the blastula of the Amphibia, the cleft separating the two layers being equivalent to the blastocoele" (p. 13). The formation of the hypoblast (pri- mary entoderm) by a process of delamination from the upper layer or epiblast agrees with the observations by Peter ('38) on the developing chick and pigeon blastoderm (fig. 172) and of Spratt ('46) on the chick. It also agrees with some of the oldest observations, concerning the matter of entoderm formation, going back to Ollacher in 1869, Kionka, 1894, and Assheton, 1896. Others, such as Duval (1884, 1888) in the chick, and Patterson ('09) in the pigeon, have ascribed the formation of the primary entoderm to a process of invagi- nation and involution at the caudal margin of the blastoderm, while Jacobson ('38) came to the conclusion that the entoderm of the pellucid area arose in chick and sparrow embryos through a process of outgrowth of cells from the primitive plate and from an archenteric canal produced by an inward bend- ing of the epiblast and primitive plate tissue. The latter author believed that the entoderm of the area opaca arose by delamination. The hypoblast of the chick gives origin to most of the tissue which lines the future gut, and, therefore, may be regarded as the potential entodermal area. As in the amphibia and Amphioxus, the epiblast is composed of sev- eral, presumptive organ-forming areas (fig. 173A). (See Pasteels, '36c; Spratt, '42, '46.) At the caudal part of the epiblast is an extensive region of presumptive mesoderm bisected by the midplane of the future embryonic axis. Just anterior to this region and in the midplane is the relatively small, presu
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