In the next stage, on the sixth day after impregnation (fig. 61), there is a great advance in development. The embryo is considerably longer, and a great number of mesoblastic somites are visible. The body is now laterally compressed and raised from the yolk. The region of the head is more distinct, and laterally two streaks are visible (_br.c_), which, by comparison with the Sturgeon, would seem to be the two first visceral clefts[39]: they are not yet perforated. In the lateral regions of the trunk the two segmental ducts are visible in surface views (fig. 61, _sd_) occupying the same situation as in the Sturgeon. Their position in section is shewn in fig. 62, _sg_. [39] I have as yet been unable to make out these structures in section. [FIG. 62. SECTION THROUGH THE TRUNK OF A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY AFTER IMPREGNATION. _mc._ medullary cord; _ms._ mesoblast; _sg._ segmental duct; _ch._ notochord; _x._ subnotochordal rod; _hy._ hypoblast.] With reference to the features in development, visible in sections, a few points may be alluded to. The optic vesicles are very prominent outgrowths of the brain, but are still solid, though the anterior cerebral vesicle has a well-developed lumen. The auditory vesicles are now deep pits of the nervous layer of the epiblast, the openings of which are covered by the epidermic layer. They are shewn for a slightly later stage in fig. 63 (_au.v_). There is now present a subnotochordal rod, which develops as in other types from a thickening of the hypoblast (fig. 62, _x_). [FIG. 63. SECTION THROUGH THE HEAD OF A LEPIDOSTEUS EMBRYO ON THE SIXTH DAY AFTER IMPREGNATION. _au.v._ auditory vesicle; _au.n._ auditory nerve; _ch._ notochord; _hy._ hypoblast.] In an embryo of the seventh day after impregnation, the features of the preceding stage become generally more pronounced. The optic vesicles are now provided with a lumen (fig. 64), and have approached close to the epidermis. Adjoining them a thickening (_l_) of _the nervous layer_ of the epidermis has appeared, which will form the lens. The cephalic extremity of the segmental duct, which, as shewn in fig. 61, is bent inwards towards the middle line, has now become slightly convoluted, and forms the rudiment of a pronephros (head-kidney). [FIG. 64. SECTION THROUGH THE FRONT PART OF THE HEAD OF A LEPIDOSTEUS EMBRYO ON THE SEVENTH DAY AFTER IMPREGNATION. _al._ alimentary tract; _fb._ thalamencephalon; _l._ lens of eye; _op.v._ optic vesicle. The mesoblast is not represented.] During the next few days the folding off of the embryo from the yolk commences, and proceeds till the embryo acquires the form represented in fig. 65. Both the head and tail are quite free from the yolk; and the embryo presents a general resemblance to that of a Teleostean.