Section through the undivided cerebral vesicle in a chick embryo of the fourth day. The densely-packed single-layered vesicle wall is completely enclosed by the delicate mesodermal sheath. [Low-power photomicrograph.] Frontal section through cerebral vesicle of a chick at eight days' incubation. The vesicle has become subdivided by the median sulcus into hemispheres, between which extends the pia-arachnoid. The latter has already split off from the dura over the summits of the hemispheres. The continuous internal chain of vascular lumina of the pia-arachnoid is conspicuously seen closely overlying the hemispheres. [Low-power photomicrograph.] Detail of the inner portion of the pia-arachnoid in an eight-day embryonic chick. The membrana limitans interna and the chain of pial lumina are distinctly shown. In the overlying arachnoid mesh the lumina are less frequently seen.. A large one is completed on the left, and to the right of this an elongated lumen appears in process of formation. [Pen and ink drawing. Zeiss homogen. immers., Oc. I. Tissue fixed in nitric acid (3%) and hardened in alcohol. Paraffin section at 4 micra.] |