ABSTRACT. The tectal lamination was investigated in the central part of the chick embryonic tectum. Two and 5 layers were observed above the neuroepithelium (NE) on embryonic day 6 (E6) and E8, respectively. Optic fibers extended on the surface of the tectum by E8. On E10-11, the outer tectum was composed of 2 layers, that is, a fibrous layer forming the optic fiber layer on the tectal surface and a cellular layer showing the gradient of cell density. In the inner tectum, the lamination was almost completed. On E12-13, the outer tectal layers, which showed the gradient of cell density, was divided into dark and light cellular layers. The dark cellular layer was divided into 2 layers on E14-15 and further into 4 layers (layer C-F in chick) on E18. On the other hand, the light cellular layer did not change until E18, but finally, it was divided into 2 layers (layer A and B in chick) by E20. Optic fibers reached the bottom of the outer tectum by E14 showing different densities of terminals. Stratification by optic fibers was going to step into the final stage on E18. On E20, laminations according to cytoarchitectural features and the optic fiber terminals were substantially completed. In the tectum affected by destruction of the contralateral embryonic eye (E4), some cellular layers were incompletely discriminated by differences of cell density.-KEY WORDS: bird, development, retina, retinotectal connection, tectum.The tectum shows the laminar structure divided into 6 strata, stratum opticum (SO), stratum griseum et fibrosum superficialis (SGFS), stratum griseum centrale (SGC), st r at um a l bum c e ntra l e (S A C) , st ra t um gri se um p e r i v e n t r i c u l a r e ( S G P ) , a n d s t r a t u m f i b o s u m periventriculare (SFP). The superficial part (layer A -F) of the SGFS composed of 10 layers (layer A -J) receives mainly optic fibers through the SO, the largest tectopetal fiber tract [6,9,13,15]. On the other hand, the SGC sends tectofugal fibers outside of the tectum through the SAC [2,18]. Cowan and his coworkers [4,[10][11][12] investigated comprehensively the laminar formation of the chick tectum. However, it was somewhat difficult to understand precisely the relationship between the laminar formation and retinal projections from their autoradiographic data on retinotectal connections [4,5] due to high background of silver grains. Strangely, although some excellent anatomical tools have been developed, retinotectal connections related to the laminar formation have scarcely studied in chick embryos after their studies. In the present study, therefore, we purposed to survey again, using the improved tract-tracing methods, the relationships between retinotectal projections and the laminar formation focused on the SGFS comparing to a series of studies by Cowan and his coworker [4,[10][11][12]. MATERIALS AND METHODSChicks and chick embryos (White Leghorn) were used in this study. After incubation of fertilized eggs for 4 days on the conditions of 38°C and 70% humidity in a forced-draft, a small window w...