The mesoblast of the trunk is divided anteriorly into splanchnic and somatic layers.
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.
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).