The accumulation of food-yelk in the ventral wall of the primitive gut (Figs. 85, 86) is the simple cause that converts the sac-shaped cœlom-pouches of the acrania into the leaf-shaped cœlom-streaks of the craniotes. To convince ourselves of this we need only compare, with Hertwig, the palingenetic cœlomula of the amphioxus (Figs. 80, 81) with the corresponding cenogenetic form of the amphibia (Figs. 89–90), and construct the simple diagram that connects the two (Figs. 87, 88). If we imagine the ventral half of the primitive gut-wall in the amphioxus embryo (Figs. 79–84) distended with food-yelk, the vesicular cœlom-pouches (_lh_) must be pressed together by this, and forced to extend in the shape of a thin double plate between the gut-wall and body-wall (Figs. 86, 87). This expansion follows a downward and forward direction. They are not directly connected with these two walls. The real unbroken connection between the two middle layers and the primary germ-layers is found right at the back, in the region of the primitive mouth (Fig. 87 _u_). At this important spot we have the source of embryonic development (_blastocrene_), or “zone of growth,” from which the cœlomation (and also the gastrulation) originally proceeds. Fig.92. Transverse section of the chordula-embryo of a bird (from a hen’s egg at the close of the first day of incubation). Fig. 92—Transverse section of the chordula-embryo of a bird (from a hen’s egg at the close of the first day of incubation). (From _Kölliker._) _h_ horn-plate (ectoderm), _m_ medullary plate, _Rf_ dorsal folds of same, _Pv_ medullary furrow, _ch_ chorda, _uwp_ median (inner) part of the middle layer (median wall of the cœlom-pouches), _sp_ lateral (outer) part of same, or lateral plates, _uwh_ structure of the body-cavity, _dd_ gut-gland-layer. Hertwig even succeeded in showing, in the cœlomula-embryo of the water salamander (_Triton_), between the first structures of the two middle layers, the relic of the body-cavity, which is represented in the diagrammatic transitional form (Figs. 87, 88). In sections both through the primitive mouth itself (Fig. 89) and in front of it (Fig. 90) the two middle layers (_pb_ and _vb_) diverge from each other, and disclose the two body-cavities as narrow clefts. At the primitive-mouth itself (Fig. 90 _u_) we can penetrate into them from without. It is only here at the border of the primitive mouth that we can show the direct transition of the two middle layers into the two limiting layers or primary germinal layers.