OX THE FATE OF THE MUSCLE -PLATE- 117 



the processes are the same in all regions of the body. The 

 formation of the limbs, however, 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.) 3 which repre- 

 sents the commencing nerve-trunk. The dorsal root is oval 

 in transverse section, the ganglion, which is verv lar°-e, form- 

 ing 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 outwards 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 splanchnopleuric 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, owin°- 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 develop 

 ment of the spinal nerves and muscle-plates in the Rabbit 

 embryo of seven or eight days. Both histologically and mor- 

 phologically the nerves have reached the same state of 

 development. 



In Chick embryos of three days eighteen hours there is not 

 much difference in the relative amount of the growth of the 

 nerves. The whole embryo has, however, increased in size. In 

 the trunk (fig. 5) the nerves cannot yet be traced into the 



