. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 52 FORMATION OF THE GERM LAYERS of the primary endoderm immediately below, undergo a degenera- tion, the roof cells at the tip and lateral margins of the invagination becoming continuous with the persisting portions of the primary endoderm (Figs. 27,0 and 28, B). This layer, following the envelop- ing cells in their growth over the yolk-mass, gradually surrounds that structure so that it comes to lie within the archenteron. In some turtles, on the other hand, the disappearance of the floor of
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. 52 FORMATION OF THE GERM LAYERS of the primary endoderm immediately below, undergo a degenera- tion, the roof cells at the tip and lateral margins of the invagination becoming continuous with the persisting portions of the primary endoderm (Figs. 27,0 and 28, B). This layer, following the envelop- ing cells in their growth over the yolk-mass, gradually surrounds that structure so that it comes to lie within the archenteron. In some turtles, on the other hand, the disappearance of the floor of the invagination takes place at a very early stage of the infolding, the en. Fig. 28.—Diagrams Illustrating the Formation of the Gastral Mesoderm in the Gecko. ce, Chorda endoderm; ec, ectoderm; en, secondary endoderm; en1, primary endoderm; gm, gastral mesoderm.—(Will.) roof cells only persisting to grow forward to form the dorsal wall of the archenteron. This interesting abbreviation of the process occurring in the gecko indicates the mode of development which is found in the mammalia. The existence of a prostomial mesoderm in connection with the primitive streak has already been noted, and when the invagination takes place it is carried forward as a narrow band of cells on each side of the sac of secondary endoderm. After the absorption of the ventral wall of the invagination a folding or turning in of the margins. 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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