110 A. M. PATERSON. 



II. Development of the Spinal Nerves and Limb 

 Plexuses. 



The later development of the spinal nerves naturally divides itself 

 into two parts : firstly, in relation to the limbs ; and secondly, in the 

 trunk between the limbs. In essential points the processes are the 

 same in all regions of the body. The formation of the limbs, how- 

 ever, and the peculiarities in the position of the muscle-plates, give 

 rise to certain differentiations in the arrangement of the nerves in 

 those regions. 



In Chick embryos at three days (Figs. 1 and 2), both in the trunk 

 between the limbs and in the regions where the limbs are being 

 formed, the nerves have reached the same stage of development. 

 The nerve roots, which lie within and alternate with the muscle- 

 plate, have joined together. From their fusion a slender, finger- 

 like process of cells results (N.), which represents the commencing 

 nerve trunk. The dorsal root is oval in transverse section, the 

 ganglion, which is very large, forming nearly the whole of it. The 

 nerves and their roots consist of large ovoid cells, containing often 

 two or three nuclei, the long axes of the cells being directed out- 

 wards from the cord. They stain more deeply than the mesoblast 

 cells in which they lie, and are surrounded by a slight amount of 

 feebly-stained intercellular substance. 



Six hours later (at three days six hours), the slender stalk (N., 

 Figs. 3 and 4), retaining the same position within the muscle-plate, 

 has grown downwards and outwards as far as the somato-splanchno- 

 pleuric angle (a.). It has the same relative position in the trunk and 

 in the regions of the limbs, passing between the muscle-plate and 

 cardinal vein (c. v.). But, owing to the difference of growth of the 

 muscle-plates in the two regions, it has reached its lower end in the 

 regions of the limbs (Fig. 4) ; while in the trunk, the muscle-plate, 

 having by this time entered the body wall (Fig. 3), extends farther 

 than the nerve. 



This description corresponds with the condition of development of 

 the spinal nerves and muscle-plates in the Eabbit embryo of seven or 

 eight days. Both histologically and morphologically the nerves have 

 reached the same state of development. 



In Chick embryos of three days eighteen hours there is not much 



