The ontogeny of the retina and optic tectum inAequidens portalegrensis(Hensel)

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Abstract
External features of the development of fry of the brown acara,Aequidens portalegrensis, are described. There is evidence that thyroxine is a melanophore‐concentrating hormone early in larval life. There is also evidence that adrenal corticosteroids are of importance in the growth and development of fry. The development of the retina is described. All structural components of the adult retina are present by 7 days after fertilization, when rods and the capacity for visual accommodation appear. Double cones first appear a day previously; circumstantial evidence suggests that they originate from the incomplete fission of single cones. The development of the optic tectum is similar to amphibians. The pace of tectal differentiation is slower than that of the retina. When the fry become free‐swimming (+ 6 days), the wall of the tectum comprises a thickstratum periventricularewith, external to this, a thinstratum corticalis.Thestratum corticalissubsequently increases in thickness, concomitant with the migration of individual neurones from thestratum periventriculare.The distinct stratification of the adult optic tectum is not apparent until shortly after the transition to the juvenile phase (+24 days).