clxiv NERVOUS SYSTEM. 



be at first pale and grey, but afterwards to acquire medulla and become white. This 

 change of aspect is apparent in the human embryo of the fourth or fifth month. 

 Harting considers that the fibres represent at first only naked axis-cylinders, and 

 suggests that the enclosing membranous tube and white substance are produced as 

 an excretion from the axial fibre. According to Kb'lliker's account of the growth of 

 nerve-fibres at their peripheral ends, as observed in the tail of batrachian larvae, the 

 existing fibres are prolonged by rows of fusiform cells which coalesce into pale fibres. 

 These send out fine offshoots, which may join with neighbouring fibres, or with 

 branched or stellate cells, which change into branched fibres, and in both of these 

 ways the branching and conjunction of the nerves go on. The first fibres thus gene- 

 rated (embryonal fibres, Kb'll.) virtually represent bundles of two, three, or more 

 tubular dark-bordered fibres, into which they are speedily converted ; the formation 

 of the medullary sheath proceeding outwards along the branches. 



Dr. Beale has studied the formation of cells and fibres both in embryo and adult 

 animals, and the following are the principal results of his observations. In both, 

 cells are formed from nuclei imbedded in granular matter; the new-formed cells 

 are connected one to another, and two cells thus connected withdraw from each 

 other, whilst the connecting isthmus lengthens out and becomes a fibre. The fibres 

 accordingly do not sprout out from a previously insulated simple cell, but are spun 

 out of the substance of the cell, or nucleus, with which they are connected from the 

 beginning. Ganglion-cells also increase in number by division into two or more, 

 and in this case the multiplied fibres belonging to the new cells form a bundle cor- 

 responding to the nervous stem or peduncle of the original cell. A ganglion-cell may 

 also arise from (apparently) a nucleus placed in the course of a fibre, viz., a little 

 oblong granular mass (of germinal matter, Beale) connected at each end with a fibre. 

 This body first clears up at its circumference, then deviates from the straight line, so 

 that the two portions of the fibre originally prolonged from its extremities, come to 

 be connected with it at one side, and finally, by further change in its figure, at one 

 end of it, as two fibres, whilst their continuations in the bundle from which they have 

 been, as it were, looped out, run in opposite directions. The two fibres at first proceed 

 straight from the cell, but afterwards one becomes twisted round the other, and the 

 coils increase with the age of the cell, but the cell-body dwindles as it grows older. 

 A nerve-fibre in a fasciculus may also be formed from two nuclei connected at their 

 ends which withdraw from each other, the connecting thread then lengthening out 

 into a fibre. For further details, Dr. Beale's original memoir may be consulted.* 



To the foregoing may be added the chief conclusions arrived at by von Hensen 

 from recent observations on the growing nerves in the tadpole's tail. According 

 to his account, fine nervous branches are seen running out, on the tail fin, into 

 almost imperceptibly fine filaments. The nerves are at first pale, smooth, without 

 nuclei, and represent the naked axis-cylinders, though much finer. Then nuclei appear 

 upon them, but these belong to very long-drawn fusiform cells which now really 

 inclose these axis-cylinders, and form the primitive sheath of the nerve-fibres ; but 

 this ensheathment stops before reaching the finest branches, i. e., until they grow 

 larger. Afterwards the medulla appears. According to von Hensen, nerves do not 

 grow from cells such as cells of a nervous centre outwards in the direction of their 

 branches. He thinks the mode is thus. Two nerve-cells are connected by a fibre, 

 or what may be the rudiment of many fibres ; of the cells, the one is central, the 

 other eventually becomes a peripheral terminal organ ; (as already stated, he 

 believes the cutaneous nerve-fibres to be connected with epithelium cells) ; the 

 latter, in the progress of growth and development, is withdrawn from the former, 

 and the nerve thus lengthened. Moreover, both cells may divide, and the 

 nerve or fibre splits in correspondence, so that a nerve comes to be connected 

 with several central and several peripheral cells. The peripheral cell or cells 

 may divide more extensively than the central, but as corresponding divisions 

 take place in the nerve-fibres, every peripheral cell or terminal organ maintains 

 its connection with the nervous centre. Yon Hensen remarks that the foun- 

 dation of the nervous centres and of the nerves of special sense, as well a3 

 those of the skin, is laid in the corneous layer of the blastoderm, from which 

 both of the central and peripheral cells are derived originally. It will be seen that 



* Phil. Trans. 1863. 



