THE EMBRYOLOGICAL DEVELOPMENT OF PRIMARY VISUAL CENTERS IN THE TURTLEEMYS-ORBICULARIS

The development of the primary visual centres was studied in a series of embryos of the turtle, Emys orbicularis, incubated at 25-degrees-C. The differentiation of both visual and nonvisual diencephalic and mes encephalic structures takes place entirely within the 2nd quarter of t he period of incubation; this finding appears to be consistent with pr evious descriptions of the embryology of 2 other chelonian species, Le pidochelys and Chelydra. Two successive waves of migration, each divid ing into internal and external sheaves, are involved in the formation of the structures of the diencephalon and mesencephalon. The primary v isual centres, which comprise 2 hypothalamic, 5 thalamic and 5 pretect al zones of retinal projections, together with the 2 superficial layer s of the tectum and a single tegmental projection zone, all have their origin in the external sheaf of the 1st wave of migration. The findin g that the adult nucleus geniculatus lateralis dorsalis, pars ventrali s arises from one of the migrations of the dorsal thalamus is discusse d in the context of the debate over the possible homologues of the mam malian geniculostriate visual pathway.