Neuroectodermal thickening in the lateral wall is destined to become the retina, whereas surface ectodermal thickening gives rise to the lens placode. Between E10 and E11, the lens placode first creates the lens pit and ultimately the lens vesicle. At E11.5 the lens vesicle has developed and appears as a spherical cavity with an echogenic border circumscribed by a narrow hypoechoic rim (
Fig. 3A , arrow). At this point there was no evidence of lid, cornea, or retinal development in the UBM image. On E11 and E12, the first evidence of vascular development appears in the primitive optic cup. A groove or choroidal fissure forms, providing access for the hyaloid artery to the optic cup. The hyaloid artery extends anteriorly, branching to form the vascular tunic of the lens. The tissues of the retina develop posteriorly first and subsequently peripherally in a complex series of steps. The latter vascular developments were not visible in the UBM images and did not appear to generate detectable Doppler signals, perhaps because of signal-to-noise problems, which we are investigating for future experiments. Axons from developing retinal ganglion cells extend through the choroidal fissure, forming the optic nerve. Evidence of retinal development could be seen at E13.5, when the posterior aspect of the lens vesicle exhibits a thickening of its echogenic border
(Fig. 3C) . At the same time, there is evidence of increasing posterior prominence of the hyperechoic rim. Periocular mesenchymal cells of neural crest origin are thought to give rise to the corneal and conjunctival epithelium. Evidence of corneal development was seen at E14.5
(Fig. 3D) . At that point, the anterior and posterior surfaces of the cornea was reasonably resolved with a thickness of approximately 120 μm. No additional structure in the corneal stroma was visible at this time. At E14.5, the UBM image
(Fig. 3D) showed the posterior segment of the lens vesicle splitting into three distinct layers: the developing vitreous cavity (anechoic), immediately posterior to the lens followed by the retina (echoic), and the intraretinal space (anechoic). A fourth layer consisting of the retinal pigment layer is isoechoic and appeared to be less distinguished from the surrounding tissues in the UBM images. The reason for the different backscatter levels observed between various ocular structures remains to be investigated. UBM images of the primary ocular tissues from E14.5 to E18.5 demonstrated progressive morphogenesis. In particular, UBM images of the retina showed continued development and thickening through E18.5
(Figs. 3E 3F 3G 3H) . Delineation of the optic nerve in UBM images can be ameliorated by modifying the angle at which B-mode imaging is performed. However, there was some evidence of this structure from E14.5 to E18.5, as demonstrated in
Figures 3D 3E 3F 3G 3H . The development of the lid and iris were difficult to visualize with UBM, as their echogenicity appeared to be similar to that of surrounding tissue.