fHE ANCESTRY OF THE CHOEDATA. 91 



origin of the peripheral nerves is also seen ; for those portions 

 of nervous tissue which remain in the skin consist of fibres and 

 a few cells. Into the nervous tissue tlms composed run the 

 tails of ectoderm cells, and out of them, on their inner sides, 

 run many fibres into the subjacent mesoblastic tissues. Now, 

 the fibres entering this nerve-substance on its outer side are 

 plainly sensory, or at all events afferent, and the fibres 

 passing from it on its inner side are presumably motor, or at 

 least efferent, seeing that they innervate the mesoblast. 



It is clear, then, that on the separation from the skin of a 

 cord thus composed the relations of the efferent fibres will not 

 be changed, as they still remain in contact with the mesoblast. 

 But, on the other hand, if this nerve-cord be entirely separated 

 from the skin the supply of outer or afferent fibres is cut off 

 from it, unless cords of epiblast remain to connect it with the 

 skin. Applying this reasoning to the particular case of the 

 separation of the dorsal cord, we see that the afferent 

 fibres are entering it on its dorsal side, and that the efferent 

 fibres are leaving it on its ventral side. If, then, the cord 

 sinks in from the skin, the efferent fibres coming out on the 

 ventral side to supply the muscles can still do so without 

 being gathered into cords, remaining irregular as they 

 do in Balanoglossus, but without dorsal cords connecting 

 the main cord with the skin afferent impulses could only enter 

 at the two ends which remain connected with the skin ; hence 

 I submit that it is probable that the three median cords in 

 Balanoglossus minutus, &c., are to be regarded as the 

 homologues of the dorsal roots of other Chordata. It is 

 at once evident, from the physical exigencies of the case, 

 that if the nervous system arose in this way the dorsal roots 

 were from the first sensory, and that they did not arise 

 as differentiations of roots of mixed function, as has 

 often been supposed. If this is true, then, as the cord phy- 

 logeuetically comes away from the skin from before back- 

 wards the number of these dorsal cords will increase, until 

 finally the cord lies connected all along the body with the skin 

 by a series of median dorsal cords placed at intervals. 



