A fresh organ of sense, the auditory sack, now for the first time becomes visible as a shallow pit in the external skin on each side of the hind-brain (_au.v_). The epiblast which is involuted to form this pit becomes much thickened, and thereby the opacity, indicated in the figure, is produced. The mesoblastic somites have greatly increased in number by the formation of fresh somites in the tail. Thirty-eight of them were present in the embryo figured. The mesoblast at the base of the brain is more bulky, and there is still a mass of unsegmented mesoblast which forms the tail swellings. The first rudiment of the heart (_ht_) becomes visible during this stage as a cavity between the mesoblast of the splanchnopleure and the hypoblast. The fore and hind guts are now longer than they were. An invagination from the exterior to form the mouth has appeared (_m_) on the ventral side of the head close to the base of the thalamencephalon. The upper end of this eventually becomes constricted off as the pituitary body, and an indication of the future position of the anus is afforded by a slight diverticulum of the hind gut towards the exterior, some little distance from the posterior end of the embryo (_an_). The portion of the alimentary canal behind this point, though at this stage large, and even dilated into a vesicle at its posterior end (_al.v_), becomes eventually completely atrophied. It is known as the postanal gut. In the region of the throat the rudiment of a second visceral cleft has appeared behind the first; neither of them is as yet open to the exterior. In a somewhat older embryo the first spontaneous movements take place, and consist in somewhat rapid excursions of the embryo from side to side, produced by a serpentine motion of the body. [FIG. 28*. FOUR SECTIONS THROUGH THE POSTANAL PART OF THE TAIL OF AN EMBRYO OF THE SAME AGE AS FIG. 28 F. A is the posterior section. _nc._ neural canal; _al._ postanal gut; _alv._ caudal vesicle of postanal gut; _x._ subnotochord rod; _mp._ muscle-plate; _ch._ notochord; _cl.al._ cloaca; _ao._ aorta; _v.cau._ caudal vein.] A ventral flexure of the præoral part of the head, known as the cranial flexure, which commenced in earlier stages (fig. 28 D and E), has now become very evident, and the mid-brain[19] begins to project in the same manner as in the embryo fowl on the third day, and will soon form the anterior termination of the long axis of the embryo. The fore-brain has increased in size and distinctness, and the anterior part of it may now be looked on as the unpaired rudiment of the cerebral hemispheres. [19] The part of the brain which I have here called mid-brain, and which unquestionably corresponds to the part called mid-brain in the embryos of higher vertebrates, becomes in the adult what Miklucho-Maclay and Gegenbaur called the vesicle of the third ventricle or thalamencephalon.