. The beginnings of embryonic development : A symposium organized by the Section on Zoological Sciences of the American Association for the Advancement of Science, cosponsored by the American Society of Zoologists and the Association of Southeastern Biologists, and presented at the Atlanta meeting, December 27, 1955. Embryology. 240 GASTRULA-ARRESTED EMBRYOS (1944) showed that a bit of nomial head endoderm (rider) suit- ably mounted upon the surface of a yolk endoderm substratum (base) would in due course embed itself in the latter; and he was able to demonstrate that cells of the rider transf
. The beginnings of embryonic development : A symposium organized by the Section on Zoological Sciences of the American Association for the Advancement of Science, cosponsored by the American Society of Zoologists and the Association of Southeastern Biologists, and presented at the Atlanta meeting, December 27, 1955. Embryology. 240 GASTRULA-ARRESTED EMBRYOS (1944) showed that a bit of nomial head endoderm (rider) suit- ably mounted upon the surface of a yolk endoderm substratum (base) would in due course embed itself in the latter; and he was able to demonstrate that cells of the rider transformed into flask cells as they infiltrated the cells of the base. Applying this method to hybrids, Gregg and Klein (1955) found that presumptive head endoderm riders from hybrid embryos embed in yolk en- doderm bases with as much dispatch as those from normal em- bryos. Their systems were not studied histologically, but as far as 180»' 90» 0» â / PIPIENS. HXBRIO 20 "to 60 80 HOURS DEVELOPMENT l8»C 100 Fig. 5. The angle of invagination in developing Rana pipiens and hybrid embryos. (Reconstructed from data of Moore, 1946.) their evidence goes, it sustains the belief that hybrid dorsal lip endoderm is capable of normal morphogenetic behavior. Archenteron Formation and Enlargement. Continued invagi- nation of the pigmented streak just vegetal to the earliest dorsal lip soon results in the formation of a cavity, the archenteron, which in normal amphibian embryos enlarges continuously while gastrulation proceeds (Fig. 4). Moore has devised a method for obtaining a measure of the rate and extent of archenteron forma- tion. It consists in obtaining the value of the angle subtended by two lines in the median sagittal plane, both originating at a point on the center of the blastocoel floor, and tangent respectively. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of t
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