470 THE ORIGIN OF VERTEBRATES 
predominate over the muscular or glandular, as in the formation of the 
electric organs of the Torpedo and Malapterurus, but still the final 
effect is produced by the alteration of the muscle or gland-cell. On 
the afferent side especially free nerve-terminations are largely recog- 
nized, or, as in Barker's book, nerves are spoken of as arising in 
connective tissue, Thus the numerous kinds of special sense-organs, 
such as Pacinian bodies, tendon-organs, genital corpuscles, etc., are 
all referred to by Barker under the heading of “sensory nerve 
beginnings in mesoblastic tissues.” Yet the type of these organs 
has been known for a long time-in the shape of Grandry’s corpuscles 
or the tactile corpuscles in the duck’s bill, where it has been proved 
that the nerve terminates in special large tactile cells derived from 
the surface-epithelium. 
So also with all the others, further investigation tends to put 
them all in the same category, all special sensory organs originating 
from a localized patch of surface-epithelium. Thus Anderson has 
shown me in his specimens how the young Pacinian body is 
composed of rows of epithelial cells, into each of which a twig 
from the nerve passes. He has also shown me how, in the case of 
the tendon-organ, each nerve-fibre passes towards the attachment of 
the tendon and then bends back to supply the tendon-organ, thus 
indicating, as he suggests, how the nest of epithelial cells has 
wandered inwards from the surface to form the tendon-organ. Again, 
Meissner’s corpuscles and Herbst’s corpuscles are evidently referable 
to the same class as those of Grandry and Pacini. 
Yet another instance of the same kind is to be found in the 
chromatophores of the frog and other animals which are under the 
influence of the central nervous system and yet have been supposed 
by various observers to be pigmented connective tissue cells. The 
most recent work of Leo Loeb and others has conclusively shown 
that such cells are invariably derived from the surface-epithelium. 
Finally, in fishes we find the special sense-organs of the lateral 
line and other accessory sensory organs, all of which are indisputably 
formed from modified surface epithelial cells. 
The whole of this evidence seems to me directly against Barker’s 
classification of sensory nerve-beginnings in mesoblastic tissues; in 
none of these cases are we really dealing with free nervous tissue 
alone, the starting point is always a neuro-epithelial couple. 
We may then, I would suggest, look upon the adult as formed of : 
