The American journal of anatomy . ectoderm. Hseinaluin. gap, and pushes up on the dorsal surface of the oral entoderm toabout the point of contact of preoral entoderm and mid-dorsal portion of the in-growing ectoderm gives rise to thevomerine teeth. The in-growing ectoderm forms the dental ridgesin which the teeth soon begin to make their appearance (Figs. 12-18). For the purpose of this study the formation of the teeth neednot be followed in detail. One point only is of especial interesthere, namely, that when the teeth are formed they are separated from 58 J. B. Johnston. the
The American journal of anatomy . ectoderm. Hseinaluin. gap, and pushes up on the dorsal surface of the oral entoderm toabout the point of contact of preoral entoderm and mid-dorsal portion of the in-growing ectoderm gives rise to thevomerine teeth. The in-growing ectoderm forms the dental ridgesin which the teeth soon begin to make their appearance (Figs. 12-18). For the purpose of this study the formation of the teeth neednot be followed in detail. One point only is of especial interesthere, namely, that when the teeth are formed they are separated from 58 J. B. Johnston. the moutb cavity by a layer of entoderm. The eruption of the teethinvolves the piercing of this entoderm (Figs. 14, 1G, 18, 20). 6. Appearance of Oro-Pharyngeal Cavity. The splitting of 1 lie solid entoderm takes place progressively frombehind forward at a stage after the teeth are well formed in thedental ridges (Figs. 14, 16, 17). By the time the splitting of theoral entoderm takes plaee the month region has grown wider from side. Fie. L6. A. punctatum, appearance of mouth cleft. Median sagittal sectioni lhe mouth. The ectoderm and dental ridges are more deeply shaded. Theyolk granules are all drawn and the outline between ectoderm and entodermis taken from I lie cell boundaries. Iron hematoxylin, fuchsin. to side in anticipation of the future wide mouth. The whole anteriorpart of the entoderm lias taken part in this widening, so that the(Till entoderm is now a broad flat solid plate. As the pharyngealcavity progresses toward the mouth it advances most rapidly towardilic lateral angles of the mouth. When the tooth germs begin toform there is a thin place left in the ectoderm between the maxillaryand vomerine teeth. At this place the entoderm forms an elbowdorso-cephalad which is quite pronounced at the lateral border of(lie tooth-forming areas (Figs. 14, 17, 18). In the meantime the The Limit Between Ectoderm and Entoderm. 59 nasal sacs have been formed and the wall of the nasal
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1910
