Abstract
This paper deals with a morphological and histological description of the exencephalocele (Hernia cerebri) found in a juvenile salmonoid fish, the so-called Sirasu-larva of dwarf Koayu which is landlocked in Lake Biwa, the largest lake in Japan. The present report may be the first record of spontaneous hernia cerebri in the teleostean fishes. A large quantity of a left half of optic lobe (mesencephalon) erupted through the defective portion, that is, the perforated left half of parietal bone, and formed a large swelling like a kind of sarcoma. As the swelling is not covered with skin and meninx, the protuberant part is in the naked state thoroughly. Accordingly, the arrangement and disposition of mesencephalon, particularly of cellular layer of optic tectum, granular and molecular layers of cerebellum, infundibulum and inferior lobe showed a remarkable bilateral asymmetry, however there have no detectable abnormality in the cerebrum (telencephalon) and olfactory lobe. The pituitary gland is under the condition of severe deformity, and the picture of function of thyroid follicles is more active than that of normal ones. A brief account of causality is given concerning the present peculiar specimen that has survived about two months after it was spawned.
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