. The Journal of comparative neurology and psychology. ities. Many mitoses appear here,especially in the front wall of this cavity. The preoral entodermcontinues forward beneath the infundibulum. As compared withearlier stages, before the formation of the anterior cavities the pre-oral entoderm is very slender and at one point beneath the infun- Johnston, Forebrain Vesicle in Vertebrates. 473 dibulum it is completely obliterated. In some embryos it is impos-sible to distinguisli any preoral entoderm beneath the brain in frontof the infnndibuliim, but in others there is no doubt whateverthat a
. The Journal of comparative neurology and psychology. ities. Many mitoses appear here,especially in the front wall of this cavity. The preoral entodermcontinues forward beneath the infundibulum. As compared withearlier stages, before the formation of the anterior cavities the pre-oral entoderm is very slender and at one point beneath the infun- Johnston, Forebrain Vesicle in Vertebrates. 473 dibulum it is completely obliterated. In some embryos it is impos-sible to distinguisli any preoral entoderm beneath the brain in frontof the infnndibuliim, but in others there is no doubt whateverthat a slender rod of cells lying in this position is the remnant ofthe anterior part of the preoral entoderm. This rod of cells is sur-rounded by cells (mesectoderm) derived from the neural crest abovedescribed, which constitute by far the largest part of the cells lyingin this position. This mesectoderm forms a considerable mass ofcells filling a lens-shaped space between brain and ectoderm andbetween infundibulum and neuropore and also extends as a sheet. Fig. 9. Neals Fig. 11 modifled by addition of the terminal part of theneural crest. The figure represents the lateral view of a cleared embryo ofSqualus of 24-25 somites and shows the extent of the mesectoderm derivedfrom the neural crest. along the sides of the infundibulum beneath the optic 9 is a copy of Neals Fig. 11, PI. 3, of this stage, with the addi-tion of this mesectoderm. In one embryo of 26 somites (Fig. 10) I have found the slenderrod of preoral entoderm persisting beneath the infundibulum, butfrom this time on it is impossible to recognize entoderm in thisposition. The infundibulum has become depressed until it is incontact with the ectoderm and at the same time, whether because ofpressure from the infundibulum or not, the median part of thepreoral entoderm becomes obliterated while its lateral portions form 474 Journal of Comparative Neurology and Psychology. themselves into the walls of the anterior he
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