_CH._ cerebral hemispheres; _FB._ vesicle of the third ventricle; _Op._ eyeball; _nf._ nasofrontal process; _M._ cavity of mouth; _SM._ superior maxillary process of _F. 1_, the first visceral fold (inferior maxillary process); _F. 2_, _F. 3_, second and third visceral folds; _N._ nasal pit; _ot._ otic vesicle. In order to gain the view here given the neck was cut across between the third and fourth visceral folds. In the section _e_ thus made, are seen the alimentary canal _al_, the neural canal _n.c._, the notochord _ch_, the dorsal aorta _AO_, and the vertebral veins _V_.] The third day, therefore, marks the distinct differentiation of the brain into five distinct parts: the cerebral hemispheres, the central masses round the third ventricle, the corpora bigemina, the cerebellum and the medulla oblongata; the original cavity of the neural canal at the same time passing from its temporary division of three single cavities into the permanent arrangement of a series of connected ventricles, viz. the lateral ventricles, the third ventricle, the iter (with a prolongation into the optic lobe on each side), and the fourth ventricle. By the third day the lens of the eye has become formed by an invagination of the epiblast, and other changes in the eye have taken place. The external opening of the auditory pit is closed before the completion of the third day (fig. 114, _RL_); and the rudiments of the external parts of the organ of smell have become formed as small pits on the under surface of the fore-brain (fig. 112, _N_). Like the lens and the labyrinth of the ear, they are formed as invaginations of the external epiblast; unlike them they are never closed up. [FIG. 114. SECTION THROUGH THE HIND-BRAIN OF A CHICK AT THE END OF THE THIRD DAY OF INCUBATION. _IV._ Fourth ventricle. The section shews the very thin roof and thicker sides of the ventricle. _Ch._ Notochord; _CV._ Anterior cardinal vein; _CC._ Involuted auditory vesicle; _CC_ points to the end which will form the cochlear canal; _RL._ Recessus labyrinthi (remains of passage connecting the vesicle with the exterior); _hy._ Hypoblast lining the alimentary canal; _AO._, _AOA._ Aorta, and aortic arch.] During the second and third days there are formed the visceral or branchial clefts, homologous with those of the Ichthyopsida, though never developing branchial processes from their walls. They are however real clefts or slits passing right through the walls of the throat, and are placed in series on either side across the axis of the alimentary canal, lying not quite at right angles to that axis nor parallel to each other, but converging somewhat to the middle of the throat in front (fig. 112 and fig. 113).