THE CONNECTIVE TISSUE OF THE FROG SCIATIC NERVE

1. The endoneurium in a frog nerve is less abundant than in a mammalian nerve, but as in the latter it consists of an inner and an outer endoneurium. There is no evidence that either of these forms a complete envelope around single nerve fibres. 2. The frog nerve perineurium consists of a deep, continuous layer of squamous epithelium, and a superficial, collagenous layer. It is not a lamellated structure. In addition, one or two apparently incomplete cellular layers of a different character can often be distinguished within the perineurium over the trunk and the branches of the sciatic nerve, but not over the roots. 3. The frog nerve epineurium is composed of loose connective tissue of a rather indistinct nature. A layer of endothelial cells found on the surface of the nerve in the region of the roots is probably derived from the abdominal peritoneum. 4. The perineurium prevents the quick penetration of silver nitrate into the nerve bundle, and behaves as an inextensible, semi‐permeable membrane when the nerve is placed into a hypotonic solution. 5. A solution of methylene blue injected into the nerve bundle does not escape to the surface rapidly unless a very high pressure is applied. 6. Effective desheathing involves removal of all the components of the perineurium. Cleaning of the nerve surface which removes all traces of epineural tissue does not affect the rate of penetration of electrolytes into the nerve. If the perineural epithelium is removed as well, there is a great increase in the apparent permeability of the nerve surface. 7. These facts confirm the hypothesis that the perineurium is a diffusion barrier which insulates and protects the nervous elements from the surrounding mesodermal tissues: this is in accordance with embryological evidence that the perineurium is ectodermal in origin.