626 A. A. W. nUBEEOHT. 



Rohon^s figures. This plexus does not reveal the presence of 

 any distinct lateral longitudinal nerve, nor any ganglia of 

 spinal or cephalic nerves. The latter {en) may be said to be 

 three in number. Visceral branches {vi. sy) are given oflp by 

 the dorsal nerves {dr). The ventral ones, springing from the 

 lower edge of the raedulkj are here represented as shorter 

 stems {vr). 



The opposite half of the system, seen in transparent per- 

 spective, as given in the two other figures, is purposely omitted 

 here, because of the asymmetry of Amphioxus in this 

 respect. 



Now a glance at these figures will convince us that the 

 situation of the Neraertean medullary nerve in its plexus, and 

 with its set of transverse nerves, is directly comparable to the 

 Vertebrate medulla and spinal nerves. The nerve-plexus 

 filling up the intervening spaces in Neraertea is present as a 

 transitory structure in Amphibian embryos. 



The ulterior appearance of an anterior enlargement forming 

 the Vertebrate brain ; the higher complication attained by the 

 brain and spinal cord when its mass increases, but not its 

 dorsal expansion, by the appearance of medullary ridges ; and 

 the formation of a neural canal by infolding of the neural 

 plate, all these are important developmental facts which do not 

 in any way weaken the grounds for comparison of the two 

 structures. They may be looked upon as adaptations to the 

 much more considerable efficiency and concentration that is 

 gradually attained by the central nervous system as we ascend 

 higher in the scale of the animal kingdom. 



The fact that the neural ridge in so many Vertebrata 

 precedes the appearance of the spinal nerves, and is inserted 

 along the top of the folds that bend together to form the 

 neural tube, may be thus interpreted, that during the phylo- 

 genetic process of infolding the transverse nerve-tracts (dorsal 

 spinal roots) remain attached in the same way to the medio- 

 dorsal collecting trunk as they did in the ancestral forms, and 

 are dragged upwards by the infolding process. The ventral 

 roots must be phylogenetically linked to the plexus as well ; 



