1862.] 141 



many appearances observed by the author in connexion with the peri- 

 pheral distribution of the nerves, not only in certain tissues of man 

 and the higher mammalia, but also in invertebrate animals. 



1 1 . The distribution of the finest branches of the nerve-fibres can 

 only be demonstrated in tissues which have been immersed in fluids 

 which refract highly, as syrup or glycerine. 



III. " Researches on the Development of the Spinal Cord in 

 Man, Mammalia, and Birds." By JACOB LOCKHART 

 CLARKE, Esq., F.R.S. Received May 20, 1862. 



(Abstract.) 



In the first stage of development the spinal cord consists simply 

 of a canal surrounded by a single layer of small cells or nuclei, which 

 are not distinguishable from each other in regard to size or structure, 

 and are so closely aggregated as to appear in actual contact. After 

 a time this homogeneous layer, while it increases in depth, separates 

 irregularly into two strata, the inner stratum forming the epithelium, 

 and the outer the grey substance. This differentiation of structure 

 proceeds gradually, and is not at first marked by any definite line of 

 separation, nor by any apparent difference in the structure of the 

 component cells. At the same time there is gradually formed 

 around the walls of the nuclei a granular substance, which unites 

 into processes or fibres, and constitutes a continuous network, by 

 which all the nuclei or cells of both layers are uninterruptedly con- 

 nected. In the grey layer there is at first no apparent difference 

 between the nuclei or cells of the anterior and posterior portion, 

 although in each portion dark or more closely aggregated groups 

 may be observed in connexion with roots of the nerves. As deve- 

 lopment advances, however, while the nuclei of the posterior grey 

 substance remain for a long time but little altered, those of the an- 

 terior substance increase in size, become more granular, and are con- 

 nected by thicker fibres united in a coarser network. At the same 

 time, in the separate groups of the anterior grey substance, the 

 granular network around the nuclei assumes a coarser and sponge- 

 like structure, until it constitutes a number of large roundish or irre- 

 gular and adjacent cells with thickening and nucleated walls. While 

 these are in course of development, the contained nuclei are forming 



