. The biology of the frog . of the yolkcells, but while this may be a factor in the case, it certainlycannot be the predominant one. (See Robinson and Asshe-ton 91,1 Assheton 94/ Morgan 97-3) As the archenteronincreases in size, the blastoccel or segmentation cavity neces-sarily becomes smaller. According to Marshall the formerbreaks through into the latter, and the two form one cavity. The Germ Layers.—The formation of the gastrula pro-duces a two-layered embryo, each layer being several cells 1 Robinson and Assheton, Quart four. Mic. Set., VoL 32, 1891. - Assheton, Ibid., Vol. 37, 1894. 3 Mo
. The biology of the frog . of the yolkcells, but while this may be a factor in the case, it certainlycannot be the predominant one. (See Robinson and Asshe-ton 91,1 Assheton 94/ Morgan 97-3) As the archenteronincreases in size, the blastoccel or segmentation cavity neces-sarily becomes smaller. According to Marshall the formerbreaks through into the latter, and the two form one cavity. The Germ Layers.—The formation of the gastrula pro-duces a two-layered embryo, each layer being several cells 1 Robinson and Assheton, Quart four. Mic. Set., VoL 32, 1891. - Assheton, Ibid., Vol. 37, 1894. 3 Morgan, The Development of the Frogs Egg, 1897. 96 THE BIOLOGY OF THE FROG thick. The outer of these layers is the ectoderm; the inner,the entoderm. The cells of the former are small and pig-mented ; those of the latter for the most part are compara-tively large, lighter in color, and contain a large amount ofyolk. The two layers are continuous with each other at thelips of the blastopore. Before the process of invagination. Y DP FIG. 20.— Sagittal section through a frog embryo. B, blastoccel; BP,dorsal lip of blastopore; BP, ventral lip of blastopore; EE, epidermiclayer of ectoderm ; EN, inner or nervous layer of ectoderm; //.hypo-blast or entoderm; T, mesenteron or gastrula cavity; Y, yolk plug.(After Marshall.) is completed there appears a third germ layer, the mesodermor mesoblast, between the other two. The mesoderm ap-pears all around the blastopore, and as this opening closesmainly from in front backward, the two masses of meso-derm on either side are brought near each other in the mid-dorsal line. The free ventral edges of the masses or sheets THE DEVELOPMENT OF THE FROG 97 of mesoderm extend ventrally until they meet below andcome to surround the archenteron, except for a short spacealong the dorsal side. The sheets of mesoderm soon be-come split into an inner or splanchnic layer, which lies nextto the archenteron, and an outer, parietal, or somatic layer,which lies
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