I/O DEVELOPMENT OF THE SPINAL NERVES 



canal and tube become more nearly circular in section. Im- 

 mediately beneath the neural tube is situated the notochord (<://). 

 It exhibits at this stage a central area rich in protoplasm, and a 

 peripheral layer very poor in protoplasm ; externally it is in- 

 vested by a distinct cuticular membrane. 



Beneath the notochord is a peculiar rod of cells, constricted 

 from the top of the alimentary canal 1 . On each side and below 

 this are the two aortae, just commencing to be formed, and 

 ventral to these is the alimentary canal. 



On each side of the body two muscle-plates are situated ; 

 their upper ends reach about one-third of the way up the sides 

 of the neural tube. The two layers which together constitute 

 the muscle-plates are at this stage perfectly continuous with the 

 somatic and splanchnic layers of the mesoblast, and the space 

 between the two layers is continuous with the body cavity. 

 In addition to the muscle-plates and their ventral continuations, 

 there are no other mesoblast-cells to be seen. The absence of 

 all mesoblastic cells dorsal to the superior extremities of the 

 muscles is deserving of special notice. 



Very shortly after this period and, as a rule, before a third 

 visceral cleft has become visible, the first traces of the spinal 

 nerves make their appearance. 



First Stage. The spinal nerves do not appear at the same 

 time along the whole length of the spinal canal, but are formed 

 first of all in the neck and subsequently at successive points 

 posterior to this. 



Their mode of formation will be most easily understood by 

 referring to PI. 22, figs. B I, B II, Bill, which are representa- 

 tions of three sections taken from the same embryo. B I is 

 from the region of the heart ; B II belongs to a part of the 

 body posterior to this, and B III to a still posterior region. 



In most points the sections scarcely differ from PI. 22, fig. A, 

 which, indeed, might very well be a posterior section of the 

 embryo to which these three sections belong. 



The chief point, in addition to the formation of the spinal 

 nerves, which shews the greater age of the embryo from which 

 the sections were taken is the complete formation of the aortas. 



1 Vide Balfour, " Preliminary account of the Development of Elasmobranch 

 Ki.-ihes," Quart. Journ. of Microsc. Science, Oct. 1874, p. 33. [This edition, p. 96.] 



