. The Journal of comparative neurology and psychology. ^ While in all lower classes, except cyclostomes, a prominent velumtransversum marks the boundary between diencephalon and telen-cephalon, in mammals the velum has not heretofore been large collection of pig embryos in this laboratory gives excel-. FiG. 34. Pig embryo of 5 mm. Model of right half of head seeu frommedial sm-face. The optic vesicles are still connected with the primitiveoptic groove. The Roman numerals indicate the brain neuromeres. x lent opportunity for comparison with the lower classes describedabove.
. The Journal of comparative neurology and psychology. ^ While in all lower classes, except cyclostomes, a prominent velumtransversum marks the boundary between diencephalon and telen-cephalon, in mammals the velum has not heretofore been large collection of pig embryos in this laboratory gives excel-. FiG. 34. Pig embryo of 5 mm. Model of right half of head seeu frommedial sm-face. The optic vesicles are still connected with the primitiveoptic groove. The Roman numerals indicate the brain neuromeres. x lent opportunity for comparison with the lower classes describedabove. The earliest stage available is a 5 mm. pig cut in transverse seriesfrom which a model of the brain has been made (Fig. 34). Fromthe figure it will be seen that this brain agrees very closely withthat of the Squalus embryo of about 20 somites. The cavity of theoptic stalk is continuous with a groove which traverses the medianline, the primitive optic groove. Behind this is the primitive inferiorlobe, a ventral expansion bounded caudally by the tuberculum pos-terius. In front of the primitive optic groove is a transverse ridgewhose cross section in the median plane presents the form of an arch. Johnston, Forebrain Vesicle in Vertebrates. 497 This is the terminal ridge. In front of this is a median pit, theterm
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