. The Journal of comparative neurology and psychology. j md Fig. 11. Squalus ac, 30 somites, frontal section through premandibular andanterior head cavities, x 150. is small and disappears at this stage. The premandibular somitesretain the connection with the median mass in which cavities appearin later stages and finally fuse with the cavities of the premandib-ular somites as fully described by l^eal and others. The anterior 47^ Journal of Comparative Neurology and Psychology. cavities extend forward at the sides of the infundibular region inthe position in which they were firs
. The Journal of comparative neurology and psychology. j md Fig. 11. Squalus ac, 30 somites, frontal section through premandibular andanterior head cavities, x 150. is small and disappears at this stage. The premandibular somitesretain the connection with the median mass in which cavities appearin later stages and finally fuse with the cavities of the premandib-ular somites as fully described by l^eal and others. The anterior 47^ Journal of Comparative Neurology and Psychology. cavities extend forward at the sides of the infundibular region inthe position in which they were first described by Miss Piatt. Ex-cept where they are in contact with the premandibular somite the. Fig. 12. Squalus ac, 29 somites, froutal sectiou through the premaudibiihirand anterior head cavities. These cavities are slightly more advanced indevelopment than in the 30 somite embryo shown in Fig. 11. x 150. anterior somites are almost completely ensheathed by the mesec-toderm derived from the region of the neuropore. The mesectodermsheet derived from the thalamic nerve rudiment has now come downbehind the eye and met with this which lies beneath the eye. From Johnston, Forcbrain Vesicle in Vertebrates. 477 this stage onward Neals figures 12, 13, etc., show the distributionof neural crest mesectoderm ahnost correctly. The mesectodermlying in the region between the optic vesicle and somite 1 shouldbe continued as a thin sheet between the brain and ectoderai beneaththe neuropore and the origin of all this part from the terminal partof the neural crest should be taken into account. tm-ei
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