422 Dr. II. Ludwig on the 



llie norvc-iiiig as well as the radial nerves emanating from it 

 at this stage consist solely of closely-packed cells, arranged 

 in several layers one above tl\e other. It is not until the 

 following day that beneath the cells of the nerve-ring a very 

 finely fibrillar layer is visible, the fibres of which run parallel 

 with the longitudinal axis of the nerve-ring. From the 

 thirteenth day onwards we observe isolated cells scattered 

 about at random between these fibres. AYith this the struc- 

 ture of the nerve-ring has reached a point at which it remains 

 in all subsequent stages of development examined by me. 

 It therefore consists of a superficial layer of cells {{. e. a layer 

 turned towards the exterior), and beneath this a layer of fibres 

 sheltering scattered cells. The five radial nerves resemble 

 the five radial vessels of the water-vascular system which 

 they accompany in so far as tliey differ from one another in 

 thickness and length and also develop unequally fast from a 

 histological point of view. As among the radial vessels, so 

 also in the radial nerves the median ventral one is in advance 

 of the others, and among the latter, again, the two dorsal take 

 ]ireccdence over the two ventral ones. Even on the eighth 

 day the rudiment of the median ventral nerve extends to 

 beyond the rudiment of the first two feet, and here reaches 

 somewhat further backwards than the blind end of the median 

 ventral radial vessel. The histology of the median ventral 

 radial nerve is similar to that of the nerve-ring, since on the 

 eighth day the nerve consists solely of cells, but on the ninth 

 of a layer of cells, which is merely superdcial, and of a sub- 

 jacent layer of fine longitudinal fibres. The separation of 

 this fibrous layer commences in the proximal portion of the 

 nerve, and from here gradually progresses until it reaches the 

 distal portion, though the extreme end of the nerve always 

 retains a purely cellular character in the stages which I 

 examined. In one respect only is the nerve-ring temporarily 

 in advance of the median ventral radial nerve, namely with 

 regard to the appearance of cells in the interior of the fibrous 

 layer. At the time when we meet with cells in the fibrous 

 layer of the nerve-ring {i. e. the thirteenth day) they are as 

 yet entirely wanting in that of the radial nerve. On the 

 twelfth day the separation into outer cellular and inner fibrous 

 layer can be seen in the two latcro-dorsal nerves also, while 

 the same separation in the case of the two latero-vcntral nerves 

 is not visible until the eighteenth day. Primarily the cellular 

 stratum of the radial nerves is two to three layers thick ; sub- 

 sequently, however, it is only one layer thick, and it then 

 represents the well-known external marginal cells of the 

 adult. 



