THIRTY-EIGHT FIGlJRES INTRODUCTIONThat the mesodermic somites exert a morphogcnetic role upon the development and segmentation of the spinal ganglia is a fact that has been supported by a number of experimental studies (Lehmann, '27 ; Detwiler, '32, '33 a, '33 b). Working up011 tlic embryos of Plenrodeles shortly after the closing of the neural folds, Lehmann excised the somites as well as grafted a region of the spinal cord lateral to thc host's somites. From his findings Lehmann drew essentially the following coriclusioris : 1) that in the complete absence of the somites of a definite i*egion, the developmcnt of the respective spinal ganglia is entirely suppressed (p. 102) ; 2) "that the segmental arrangement of the spinal ganglia o~w r s only wlieii therc is a degree of normal arrangement of the mesodermal structiires, and that in almost all cases disturbances in the mesoderm are accompanied by an abnormal develo1)-meiit of tlie ganglion cells"; 3) that the normal location of the sensory and motor roots is subservient to a normal arrangement of the mesoderm, and that disturbances in the latter bring about alteration in the location of the out-growing fiber bundles; 4) that it is the presence of thc mesial surface of the somites mhich brings about differentiation and segmen- iation of the ganglia. The lateral surface of tlic somite lacks this formative quality, according to Lehmann, for when the cord was grafted lateral to the sornites, segmentation in the ganglia failed and little or no differentiation of cells occurred.Similar. experiments by tlie author upon the embryos of Amblystoma ('32) have yielded a number of cases in which discrete spinal ganglia developcd and differentiated in tlie complete absence of developing muscle and cartilage-a condition which, according to Lehmann, did not exist in Pleurodelcs. Moreover, a number of eases were obtained in which spinal ganglia developed along the lateral surface of the host 's somites when tlie spinal cord WRS grafted to this region. That tlie lateral surface of the somite inhibits ganglion cell differentiation, as coiicluded by Lehmann, could not be substantiated in my experiments.From the results so far at hand, it would liardly seem justifiable for us to conclude that no differentiation of spinal ganglion cells can take place in the absence of the mesial face of the somite or of the axial skeleton regardless of the morpliogenctic influence these tissues may exert under normal conditions of development. It would appear that the crest cells possess a certain amoiint of self-differentiating capacity in the same sense that Harrison ('10) foirnd the embryonic neuroblasts to develop in vitro. It may be that tissues other than developing myotome or sclerotome maS influence differentiation, for in several of my cases ganglia developed when adjacent 1 o glandular tissue (pronephros) and the notochord.The most conviiicing evideiice supporting the view that spinal ganglia can develop and diff ercntiate in the absence of muscle and cartilage was obtained from those case...

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