. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 50 FORMATION OF THE GERM LAYERS This is not, however, the only source of the mesoderm in the am- phibia, for while the blastopore is still quite large there may be found surrounding it, between the endoderm and ectoderm, a ring of mesodermal tissue (Fig. 25, mes). As the blastopore diminishes in size and its lips come together and unite, the ring of mesoderm forms first an oval and then a band lying beneath the line of closure of the blastopore and united with both the superjacent ectoderm an
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 50 FORMATION OF THE GERM LAYERS This is not, however, the only source of the mesoderm in the am- phibia, for while the blastopore is still quite large there may be found surrounding it, between the endoderm and ectoderm, a ring of mesodermal tissue (Fig. 25, mes). As the blastopore diminishes in size and its lips come together and unite, the ring of mesoderm forms first an oval and then a band lying beneath the line of closure of the blastopore and united with both the superjacent ectoderm and the subjacent endoderm. This line of fusion of the three germ. Fig. 26.—Section through an Embryo Amphibian (Triton) of 2% Days, showing the Formation of the Gastral Mesoderm. ok, Ectoderm; ch, chorda endoderm; dk, digestive cavity; ik, endoderm; mk1 and mk2, somatic and splanchnic layers of the mesoderm. D, dorsal and V, ventral.— (Herlwig.) layers is known as the primitive streak. It is convenient to distin- guish the mesoderm of the primitive streak from that formed from the dorsal wall of the archenteron by speaking of the former as the prostomial and the latter as the gastral mesoderm, though it must be understood that the two are continuous immediately in front of the definitive blastopore. In the reptilia still greater modifications are found in the method of formation of the germ layers. Before the enveloping cells have completely surrounded the yolk-mass, a crescentic groove, resembling that occurring in amphibia, appears near the posterior edge of the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original McMurrich, J. Playfair (James Playfair), 1859-1939. Philadelphia : P. Blakiston's son & co.
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