The development of the human body; a manual of human embryology . , becomes greatly modified,the combination of embryonic and maternal structuresproducing what is termed the deciduce, owing to its beingdiscarded at birth when the parasitic mode of life is givenup. Furthermore, it has already been seen that many pecu-liar modifications of development in the human embryoresult from the inheritance of structures from more or lessremote ancestors, and among the embryonic adnexes arefound structures which represent in a more or less modi-fied condition organs of considerable functional impor-tance
The development of the human body; a manual of human embryology . , becomes greatly modified,the combination of embryonic and maternal structuresproducing what is termed the deciduce, owing to its beingdiscarded at birth when the parasitic mode of life is givenup. Furthermore, it has already been seen that many pecu-liar modifications of development in the human embryoresult from the inheritance of structures from more or lessremote ancestors, and among the embryonic adnexes arefound structures which represent in a more or less modi-fied condition organs of considerable functional impor-tance in lower forms. Such structures are the yolk-stalk 128 THE AMNION. 129 and vesicle, the amnion, and the allantois, and for theirproper understanding it will be well to consider brieflytheir development in some lower form, such as the the time when the embryo of the chick begins to beconstricted off from the surface of the large yolk-mass, afold, consisting of ectoderm and somatic mesoderm, arisesjust outside the embryonic area, which it completely sur-. Fig. 62.—Diagrams Illustrating the Formation of the Amnicn and Allantois in the , Amnion folds; Al, allantois; Am, amniotic cavity; Ds, yolk-sac. —(Gegenbaur.) rounds. As development proceeds the fold becomeshigher and its edges gradually draw nearer together overthe dorsal surface of the embryo (Fig. 62, A), and finallymeet and fuse (Fig. 62, B), so that the embryo becomesenclosed within a sac, which is termed the amnion and isformed by the fusion of the layers which constituted theinner wall of the fold. The layers of the outer wall of the 130 THE DEVELOPMENT OF THE HUMAN BODY. fold after fusion form part of the general ectoderm andsomatic mesoderm which make up the outer wall of theovum and together are known as the serosa, correspondingto the chorion of the mammalian embryo. The spacewhich occurs between the amnion and the serosa is a por-tion of the extra-embryonic ccelom and is continuous withthe embry
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
