. The Journal of comparative neurology and psychology. 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 head cavities. The mesen-chymatous cells beneath the front end of the brain are
. The Journal of comparative neurology and psychology. 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 head cavities. The mesen-chymatous cells beneath the front end of the brain are in over-whelming proportion of ectodermal (neural crest) origin. In Dohms beantiful plates illustrating his articles on the man-dibular and premandibular cavities, this terminal neural crest isclearly shown. See especially PI. 9, Figs. 7-12. Dohrn does notdistinguish between premandibular mesoderm and the mesectodermof neural crest origin. The latter he calls the anterior part of thepremandibular mesoderm (Prsem. 1) after the infundibulum has nf en. Fig. 10. Squalus ac, 2G somites, frontal section, inf., the so-called infun-(libulum. pressed down to meet the ectoderm. In Fig. 8 the origin of thisfrom the neuropore is strongly suggested, especially as the spotmarked neurop. is at the upper border of the lamina distance dorsal to the extreme point to which the preoral ento-derm or premandibular mesoderm ever reaches. In Dr. Nealspreparations which I have studied the entoderm has a differenttone from the other tissues and there is a decided difference in theform of the cells and in the size of the nuclei between the preman-dibular mesoderm and the terminal neural crest mesectoderm. In embryos of 29 and 30 somites (Figs. 11 and 12) premandibular Johnston, Forehrain Vesicle in Vertebrates. 475 and anterior head cavities are small, sejDarate from each other, butboth connected with the median mass which is crowded behind theinfundibnlnm. The median band connecting- th
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