Functional Development of Visual Pathways in Larval Sardines and Anchovies1

One of the principal purposes of studying the nervous system is t o correlate the observed gross and microscopic structure with the dynamic activity of the animal, that is, its behavior patterns. Coordinated swimming movements, feeding, and schooling can be observed in larval fish as soon as the neural pathways, sensory and motor, are sufficiently developed. On the other hand, mating and spawning behavior, which frequently involve extensive migratory movements, require sexual maturity and stimulation from endocrine glands. This paper describes centers and fiber tracts in the central nervous system of sardine and anchovy which are involved in relaying visual information. Evidence is presented concerning the time during larval development when these morphological features make their first appearance. Virtually nothing is known about the developing nervous system in sardine and anchovy larvae, and no detailed study of the optic system of these fish exists. Therefore, extensive larval and some adult material of the two species was examined for the present paper. The species used were Xardinops caerulea (Girard) and Engraulis mordax (Girard) . Isolated brains of adult specimens, fixed in ten percent neutralized formalin, were cut a t 10 micra in transverse and sagittal serial sections; all sections were mounted and stained by the Kluver-Barrera (1953) or by the Ziesmer-Bodian (1952) method. Different methods of silver impregnation were tried on larval stages. Best results were obtained with the Ramon-Pyridine (1911) and the Bodian (Ziesmer, 1952) methods.