Experiments upon the Segmentation of Spinal Nerves in Salamander Embryos

superficiale, represented in amphibians and highly developed as a single stratum in the reptilian and avian optic tecta, is differentiated in mammals into a superficial portion to which in man the name stratum zonale and stratum cinereum are generally applied, and a deeper portion commonly known as the stratum lemnisci. Consequent to a decrease in the afferent systems there is noted a decrease in the tecto-thalamic paths. The efferent paths to motor centers carry progressively longer fibers in passing from lower to higher forms, so that in man tectospinal paths have been traced to at least the middle of the spinal cord. Thus in this sense the efferent systems of the optic tectum are more efficient in higher mammals and in man than in the lower mammals. The marked reduction of the stratum periventriculare fibrosum in higher mammals and man appears to be associated in part at least with the reduction of the periventricular fiber system. It must be evident from what has preceded that the optic tectum affords a classic example of the interrelation of structure and function. Its rise to a position of functional dominance finds its expression in a high degree of morphologic differentiation; its decline from a dominant position is associated with a decrease both in relative size and in morphologic differentiation.