Experimental Studies Upon the Development of the Amphibian Nervous System

Summary. An account is given of certain experiments which bear upon the problems of morphogenesis in the central and peripheral nervous system. By means of grafting embryonic limbs and other rudiments it has been possible to alter the direction of growth in spinal nerves. These responses are discussed in relation to the probable nature of forces responsible for the general growth and connection of nerves. Experiments involving limb and somite excision and grafting are described as they bear upon the development of primary somatic sensory and motor neurones, and upon the segmentation of spinal nerves. The results indicate a fundamental difference in the growth response of motor and sensory neurones to peripheral changes. They also support the theme that segmentation in the spinal cord is entirely subservient to mesodermic metamerism. The autoplastic interchange of various embryonic spinal cord segments, as well as heterotopic grafting experiments, are described as they bear upon the problem of cellular proliferation and differentiation within the embryonic spinal cord. These results, in combination with those following limb and somite excision and grafting, indicate that forces within the central nervous system are chiefly responsible for proliferation therein, rather than growth influences from the periphery.