The American journal of anatomy . ance of Mouth Plate Ectoderm. In Amblystoma no stomodaeum is formed. Instead, the ectodermover the mouth plate area disappears, leaving the solid entodermexposed on the free surface. Later the mouth cavity appears as acleft in this entoderm. The well known arrangement of the cells of the ectoderm in twolayers is clearly marked in the mouth plate area of Amblystoma inthe neural plate stage. As the neural tube rolls up the ectoderm The Limit Between Ectoderm and Entoderm. 55 becomes somewhat thinner, but not as thin as that covering theventral surface of the yol
The American journal of anatomy . ance of Mouth Plate Ectoderm. In Amblystoma no stomodaeum is formed. Instead, the ectodermover the mouth plate area disappears, leaving the solid entodermexposed on the free surface. Later the mouth cavity appears as acleft in this entoderm. The well known arrangement of the cells of the ectoderm in twolayers is clearly marked in the mouth plate area of Amblystoma inthe neural plate stage. As the neural tube rolls up the ectoderm The Limit Between Ectoderm and Entoderm. 55 becomes somewhat thinner, but not as thin as that covering theventral surface of the yolk mass. Following this the inner cells ofectoderm disappear from the month plate. In either transverse orsagittal sections the entoderm is seen to be covered by the thin Hatcells of the outer layer of ectoderm, while the inner layer of morecnboidal cells stops abruptly around the borders of the month plate(Figs 5 6 13). Anteriorly the month plate is bounded by diehypophysis. I have not been able thus far to see in detail how this. Fig 13 Detail drawing of the mouth plate and dental ridges in the sectiondlwnn Fig 12. Here it is seen that the outer layer of ectoderm is partlybroken down but still covers the entoderm. condition comes about. Whether the inner cells of ectoderm migrateapart and draw away from the month area or whether some of thesecells disintegrate in situ is not wholly clear. I have seen no signsof disintegrating cells, and there is to be seen a heaping up or thick-ening of the inner layer of ectoderm around the month plate. 1 amstrongly inclined to think that the inner cells draw away from themonth plate in preparation for the formation of the dental ridgesto be described below. Certain it is that by the time the hypophysisbegins to be actively invaginated (Fig. 6) the ectoderm of the month 56 J. I!. Johnston. plate consists of the thin outer layer alone. As development goeson this thin outer layer gradually breaks down and its cells seem toscale off, leaving the
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1910
