One of the most unfortunate errors that this led to was the idea of an original antithesis of germ and yelk. The latter was regarded as a foreign body, extrinsic to the real germ, whereas it is properly a part of it, an embryonic organ of nutrition. Many authors said there was no trace of the embryo until a later stage, and outside the yelk; sometimes the two-layered embryonic disk itself, at other times only the central portion of it (as distinguished from the germinative area, which we will describe presently), was taken to be the first outline of the embryo. In the light of the gastræa theory it is hardly necessary to dwell on the defects of this earlier view and the erroneous conclusions drawn from it. In reality, the first segmentation-cell, and even the stem-cell itself and all that issues therefrom, belong to the embryo. As the large original yelk-mass in the undivided egg of the bird only represents an inclosure in the greatly enlarged ovum, so the later contents of its embryonic yelk-sac (whether yet segmented or not) are only a part of the entoderm which forms the primitive gut. This is clearly shown by the ova of the amphibia and cyclostoma, which explain the transition from the yelk-less ova of the amphioxus to the large yelk-filled ova of the reptiles and birds. Fig.105. Severance of the discoid mammal embryo from the yelk-sac, in transverse section (diagrammatic). Fig. 105—Severance of the discoid mammal embryo from the yelk-sac, in transverse section (diagrammatic). _A_ The germinal disk (_h, hf_) lies flat on one side of the branchial-gut vesicle (_kb_). _B_ In the middle of the germinal disk we find the medullary groove (_mr_), and underneath it the chorda (_ch_). _C_ The gut-fibre-layer (_df_) has been enclosed by the gut-gland-layer (_dd_). _D_ The skin-fibre-layer (_hf_) and gut-fibre-layer (_df_) divide at the periphery; the gut (_d_) begins to separate from the yelk-sac or umbilical vesicle (_nb_). _ E_ The medullary tube (_mr_) is closed; the body-cavity (_c_) begins to form. _F_ The provertebræ (_w_) begin to grow round the medullary tube (_mr_) and the chorda (_ch_): the gut (_d_) is cut off from the umbilical vesicle (_nb_). _H_ The vertebræ (_w_) have grown round the medullary tube (_mr_) and chorda; the body-cavity is closed, and the umbilical vesicle has disappeared. The amnion and serous membrane are omitted. The letters have the same meaning throughout: _h_ horn-plate, _ mr_ medullary tube, _hf_ skin-fibre-layer, _w_ provertebræ, _ch_ chorda, _c_ body-cavity or cœloma, _df_ gut-fibre-layer, _dd_ gut-gland-layer, _d_ gut-cavity, _nb_ umbilical vesicle.