The American journal of anatomy . ion of other structures in the region of the inferiorLobes takes place considerably later. The shifting caudad of the preoral entoderm and hypophysis whichI have attributed to the brain flexure is accompanied by a very con-siderable elongation of the mouth entoderm. While the shifting isInking place the mouth entoderm is compressed into a solid mass,and this becomes elongated into a comparatively slender cord, flat-tened dorso-ventrally (Figs. 10, 12, 14, 16). For some time after its separation from the ectoderm the hypophy-sis continues to have the form of a
The American journal of anatomy . ion of other structures in the region of the inferiorLobes takes place considerably later. The shifting caudad of the preoral entoderm and hypophysis whichI have attributed to the brain flexure is accompanied by a very con-siderable elongation of the mouth entoderm. While the shifting isInking place the mouth entoderm is compressed into a solid mass,and this becomes elongated into a comparatively slender cord, flat-tened dorso-ventrally (Figs. 10, 12, 14, 16). For some time after its separation from the ectoderm the hypophy-sis continues to have the form of a shallow cup with the convexity The Limit Between Ectoderm and Entoderm. 53 against the brain, the concavity toward the entoderm (Fig. 9). Itfinally forms a somewhat ovoid mass and lies beneath the further history does not concern us in this study. 3. Coalescence of the Walls of the Foregut. It has already been indicated that the formation of the neural tubebegins to compress the anterior part of the archenteron. This is. Fig. 11. A diagram of the relations of the palseostoma and neostoma in A camera drawing was made from the section from which was drawn and the cleft connecting the archenteron with the hypophysiswas made wider. This is the only diagrammatic feature of the drawing. Thepala>ostoma is represented by the cleft hy- p. carried further by the formation of the mesodermic somites aboveand at the sides and of the heart below, and the bending down ofthe brain completes the influences which result in the complete oblit-eration of the cavity of the anterior part of the future mouth (, 14). This condition is well known in amphibian embryos andrequires little comment. Two things should be noted, (a) Thecoalescence of the walls begins early and is followed by a relatively 54 J. B. Johnston. greal elongation of the pharyngeal region. The necessity of this tomake room for the branchial apparatus, heart and brain, is clearfrom Figs. :;. !) a
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