26o NERVOUS SYSTEM. [SECT. 125. 



other parts. Still, in the oldest tadpoles, it is seen, that the 

 nerves in question are somewhat thicker than at first, and that | 

 they terminate at the periphery, partly in the form of loops, partly 

 with free ends, but in such a manner, that the primitive pale 

 fibres are still present, and, proceeding from the dark-bordered 

 ones, form an extremely fine ultimate nervous ramification with j 

 anastomoses and free extremities. 



I should not have dwelt so long upon the nerves of the frog, 

 were it not extremely probable that the facts observed will apply 

 to numerous other nerve-terminations. This is certainly the case 

 in those in the electric organ of the ray, which, even when fully 

 developed, agree in many respects with those of well grown tadpoles, 

 and, as Ecker has recently shown (Zeitschrift filr Wiss. Zoologie, j 

 1849, p. 38 j are developed in exactly the same manner. The 

 nerves in the skin of the mouse also obviously belong to this cate- 

 gory; and, perhaps, it may hereafter be found, that, wherever 

 peripheral nerve-divisions exist, their development takes place in 

 essentially the same manner as I have here described. 



But few investigations have been made with reference to the ! 



development of the nerve-fibres in the central organs. Of those of j 



the ganglia I can only say this much, that they are later in being de- j 



veloped than those of the nerves, and, probably, from small fusiform 



cells, which are seen among the ganglion-globules. I once saw, in 



a spinal ganglion of a human embryo of four months, such a 



fusiform cell in connection with the process of a ganglionic 



globule. The development of the fibres in the cord and brain is 



extremely difficult to investigate ; and it is best to make use of 



chromic acid preparations. In the human embryo, 1 find the 



formation of the tubes in question already commenced at the end 



of the second month, the white substance being distinctly striated, 



and containing, at different places, evidently fusiform, very delicate 



cells, which are partly isolated, partly connected, two, three, or 



several together. All these cells are at first pale, closely surround a 



nucleus, crooi"' to 0003'" in size, and have processes which are 



nearly as fine as the fibrils of connective tissue. In the fourth 



month, when the difference between the two kinds of substances 



is very distinct, we still, in parts, recognise nuclei in the fibres 



which have now become broader ; at other parts, the nuclei have 



disappeared, although the fibres have not simultaneously acquired 



dark contours, which in fact are only developed after the middle 



of foetal life, and first in the cord. 



Respecting the subsequent changes of the nerve-tubes, it has 



