1 82 DEVELOPMENT OF THE SPINAL NERVES 



The nerves correspond in number with the muscle-plates, 

 and each arises from the spinal cord, nearly opposite the middle 

 line of the corresponding muscle-plates (figs. G I and G 2). 



Each nerve- rudiment is surrounded by connective -tissue 

 cells, and is separated from its neighbours by a considerable 

 interval. 



At its origin each nerve-rudiment lies opposite the median 

 portion of a muscle-plate (figs. G I and G 2) ; but, owing to the 

 muscle-plate acquiring an oblique direction, at the level of the 

 dorsal surface of the notochord it appears in horizontal sections 

 more nearly opposite the interval between two muscle-plates 

 (figs. G 2 and G 3). 



In horizontal sections I find masses of cells which make 

 their appearance on a level with the ventral surface of the 

 spinal cord. I believe I have in some sections successfully 

 traced these into the spinal cord, and I have little doubt that 

 they are the anterior roots of the spinal nerves ; they are op- 

 posite the median line of the muscle-plates, and do not appear 

 to join the posterior roots (vide fig. G 3, ar). 



At the end of this period or second stage the main cha- 

 racters of the spinal nerves in Pristiurus are the following : 



(1) The posterior nerve-rudiments form somewhat wedge- 

 shaped masses of tissue attached dorsally to the spinal cord. 



(2) The cells of which they are composed are typical undif- 

 ferentiated embryonic cells, which can hardly be distinguished 

 from the connective-tissue cells around them. 



(3) The nerves of each pair no longer meet above the 

 summit of the spinal canal, but are independently attached 

 to its sides. 



(4) Their dorsal extremities are probably united by com- 

 missures. 



(5) The anterior roots have appeared ; they form small 

 conical projections from the ventral corner of the spinal cord, 

 but have no connexion with the posterior rudiments. 



The Tliird Stage of the Spinal Nerves in Pristiurus. 



With the third stage the first distinct histological differen- 

 tiations of the nerve-rudiments commence. Owing to the 



