A system of obstetrics . the zona pellucida being omitted ; ag isagain the germinal area, and ge points to the limit of the extension ofthe hypoblast beneath the non-embryonic epiblast at this epoch. In the embryonic <t germinal area a primitive streak and groovenext make their appearance, much as in the area pellucida of the devel-oping bird embryo (Fig. 24). As seen in the rabbit, these structuresare represented in Figs. 46 and In order to understand the mode of segmentation of the ovum of highermammals and the meaning of the blastodermic vesicle, we must bear inmind that the earlies
A system of obstetrics . the zona pellucida being omitted ; ag isagain the germinal area, and ge points to the limit of the extension ofthe hypoblast beneath the non-embryonic epiblast at this epoch. In the embryonic <t germinal area a primitive streak and groovenext make their appearance, much as in the area pellucida of the devel-oping bird embryo (Fig. 24). As seen in the rabbit, these structuresare represented in Figs. 46 and In order to understand the mode of segmentation of the ovum of highermammals and the meaning of the blastodermic vesicle, we must bear inmind that the earliest and ancestral mammals were oviparous—that they 1 The primitive streak and groove of embryonic reptiles, birds, and mammals areremnants of an ancestral structure, the blastopore. (See Balfour: Oomp. Embryol^ vol. ii. pp. Is7 and 2 THE EARLY STAGES OF DEVELOPMEST. 123 laid eggs containing much food-yelk and much like the eggs of reptilesand birds. The lowest existing Mammalia (echidna, ornithorhynchus) Fig. 46. Fig. 41.
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectobstetrics, bookyear1
