An electronmicroscopic study of developing amphibian ectoderm.

1. Small pieces of ectoderm were excised from gastrulae, neurulae, and tailbud embryos ofXenopus laevis andTriturus alpestris, preserved inLEHMANN's fixatives, sectioned at 0.025-0.75μ, and photographed with a Trüb-Täuber electron microscope. 2. The following features characterize the early gastrular ectoderm: endoplasmic reticulum coarse, vesicular, and predominantly loose; mitochondria mostly globular and irregular; lipoid droplets and yolk-platelets with investing plasma membranes; pigment granules ofTriturns, but not ofXenopus, composed of subunits; nuclei polymorphic, especially inTriturus, with deep infoldings of nuclear membrane; cells frequently connected only by cytoplasmic bridges which may be anastomosed, cells otherwise separated by spaces or canals. 3. Presumptive medullary plate from the very early neurula shows certain differences from the above features: endoplasmic and nuclear reticula considerably more dense; cytoplasmic vesicles, fibers, and granules more delicate; mitochondria smooth and rod-like with increased number of cristae; intercellular spaces less prominent, except between presumptive neural plate and chordamesoderm, the cytoplasmic processes of which may anastomose. 4. Presumptive epidermis of the very early neurula shows a wide meshed fibrous reticulum and distally situated mitochondria, foreshadowing the development of highly differentiated outer zones in the epidermal cells of the tailbud embryo. 5. Mitochondria of neural cells of the late tailbud embryo are predominantly perinuclear, quite elongate, relatively narrow, and possess many thin longitudinally or diagonally placed cristae. 6. Mitochondria of the tailbud epidermis are shorter and thicker than those of the neural tube cells, exhibit pores and swollen tubules, and occur in large numbers below a distal zone of dense cytoplasmic reticulum and secretory vesicles, which seem to form in waves and to discharge periodically their secretion to the surface of the skin. 7. The concept of mitochondrial "differentiation" is further developed and the function of epidermal mitochondria and the importance of intercellular contacts, especially in relation to neural induction, are discussed.