Fig.54. Discoid gastrula (discogastrula) of a bony fish. Fig. 54—Discoid gastrula (_discogastrula_) of a bony fish. _e_ ectoderm, _i_ entoderm, _w_ border-swelling or primitive mouth, _n_ albuminous globule of the nutritive yelk, _f_ fat-globule of same, _c_ external membrane (ovolemma), _d_ partition between entoderm and ectoderm (earlier the segmentation-cavity.) Essentially different from the wide-mouthed discoid gastrula of most of the selachii is the narrow-mouthed discoid gastrula (or _ epigastrula_) of the amniotes, the reptiles, birds, and monotremes; between the two—as an intermediate stage—we have the _amphigastrula_ of the amphibia. The latter has developed from the amphigastrula of the ganoids and dipneusts, whereas the discoid amniote gastrula has been evolved from the amphibian gastrula by the addition of food-yelk. This change of gastrulation is still found in the remarkable ophidia (_Gymnophiona, Cœcilia,_ or _Peromela_), serpent-like amphibia that live in moist soil in the tropics, and in many respects represent the transition from the gill-breathing amphibia to the lung-breathing reptiles. Their embryonic development has been explained by the fine studies of the brothers Sarasin of _Ichthyophis glutinosa_ at Ceylon (1887), and those of August Brauer of the _Hypogeophis rostrata_ in the Seychelles (1897). It is only by the historical and comparative study of these that we can understand the difficult and obscure gastrulation of the amniotes. Fig. 55 Longitudinal section through the blastula of a shark. Fig. 55—Longitudinal section through the blastula of a shark (_Pristiuris_). (From _Ruckert._) (Looked at from the left; to the right is the hinder end, _H,_ to the left the fore end, _V._) _B_ segmentation-cavity, _ kz_ cells of the germinal membrane, _dk_ yelk-nuclei.