The development of the human body; a manual of human embryology . 26.—Longitudinal Sections through Embryos of the Gecko, showing Gastrulation. ec, Ectoderm; en, secondary endoderm; en, primary endoderm; prm, prostomial mesoderm.—{Will.) it comes to lie within the archenteron. In some turtles,on the other hand, the disappearance of the floor of theinvagination takes place at a very early stage of theinfolding, the roof cells only persisting to grow forward toform the dorsal wall of the archenteron. This interest-ing abbreviation of the process occurring in the geckoindicates the mode of develo
The development of the human body; a manual of human embryology . 26.—Longitudinal Sections through Embryos of the Gecko, showing Gastrulation. ec, Ectoderm; en, secondary endoderm; en, primary endoderm; prm, prostomial mesoderm.—{Will.) it comes to lie within the archenteron. In some turtles,on the other hand, the disappearance of the floor of theinvagination takes place at a very early stage of theinfolding, the roof cells only persisting to grow forward toform the dorsal wall of the archenteron. This interest-ing abbreviation of the process occurring in the geckoindicates the mode of development which is found in themammalia. ;o THE DEVELOPMENT OF THE HUMAN BODY. The existence of a prostomial mesoderm in connectionwith the primitive streak has already been noted, andwhen the invagination takes place it is carried forward asa narrow band of cells on each side of the sac of secondaryendoderm. After the absorption of the ventral wall ofthe invagination a folding or turning in of the margins ofthe secondary endoderm occurs (Fig. 27) whereby its. Fig. 27.—Diagrams Illustrating the Formation op the Gastral Mesoderm in the Gecko. ce, Chorda endoderm; ec, ectoderm; en, secondary endoderm; en1, primary endoderm; gm, gastral mesoderm.—(Will.) lumen becomes reduced in size and it passes off on eachside into a double plate of cells which constitute the gas-tral mesoderm. Later these plates separate from thearchenteron as in the lower forms. All the prostomialmesoderm does not, however, arise from the primitivestreak region, but a considerable amount also has its originfrom the ectoderm covering the yolk outside the limitsof the blastoderm proper, a mode of origin which servesto explain the phenomena later to be described for themammalia. THE FORMATION OF GERM LAYERS. 71 In comparison with the amphibians and Amphioxus,the reptilia present a subordination of the process of in-vagination in the formation of the endoderm, a primaryendoderm making its appearance independe
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
