THE ANCESTRY OF THE CHORDATA. 559 
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 thus 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- 
logenetically 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. 
