The development of the nervous system in chick embryos, studied by electron microscopy.

Tissue from 73 chick embryos incubated for periods between 1 and 13 days has been studied. Before fixation each specimen was staged according to the normal table of Hamburger & Hamilton (1951). The tissue was fixed either in osmium tetroxide, or in potassium permanganate (Luft, 1956). In either case the pH was buffered to 7-4 with sodium veronal (Palade, 1952) and fixation was at 0° C. In the early stages the whole embryo was fixed. In the later stages the nerve-cord was dissected out partially before fixation, and completely afterwards. Dehydration was with alcohols, and the specimens were then embedded either in the epoxide resin 'Araldite' (Glauert, Rogers, & Glauert, 1956) or in methacrylate. Sections were examined in a Siemens Elmiskop IB electron microscope. Twenty-four specimens have also been studied by light microscopy. Twelve of these were stained with silver (Bielschowsky's method) and twelve were selectively stained for Nissl substance with dahlia, cresyl violet, or neutral red.