590 The Sensory Organs. [ September, 
(A. f. M. A. xii), as at least a possible, if not a probable method, 
of explaining the touch-bodies ; z. e., the corpuscles of Krause, of 
Meisner and Wagner and the Pacinian bodies. Koelliker tells us 
that they are all probably modifications of one and the same thing, 
as intermediate forms exist all over the body. Now Meckel de- 
scribes cells as existing in birds and mammals into which he 
directly traces nerve-fibres. He designates them as “ touch-cells” 
and describes their transition into touch bodies. The touch-cells 
are situated ordinarily deneath the epithelium, but in the bill of 
the pigeon they present every transitional stage between cells en- 
tirely buried in the sub-epithelial tissue, cells partially extending 
into the epithelium, and cells éntirely enclosed by the epithelium. 
Again, in the snout of the hog the “touch-cells.” are a% in the 
epithelium, though in its lowest layers. They are larger than nerve- 
cells generally, and frequently two or more become joined so as to 
form twin cells or a more complex arrangement. These last are 
found deneath the epithelium. Then he also describes structures 
made up of a larger number of cells, but in which the nerve-fibre 
still supplies each individual cell. Then he describes others 
which are still more complex and in which the separation into 
cellular elements is still less apparent. All the complex forms 
are beneath the epithelium, only the single cell is found in the 
latter. 
If the results of Meckel are correct, we have a link between the 
sensory structures of the epithelium and the touch-bodies, and 
the latter might be looked upon as epithelial structures in reality, 
but which have become displaced in the progress of development. 
The legitimacy of such an inference, however, would be rendered 
highly doubtful by the existence of the Pacinian corpuscle in the 
mesentery of the cat. To these last structures Arndt (Virch. A. 
lxv, 1875) ascribes a very peculiar significance, but which is 
_ hardly satisfactory. From observations on foetal cats he comes to 
the conclusion that the Pacinian corpuscle is an outgrowth of 
. the vascular system of the mesentery. 
Let us now turn our attention to a field that has been thus far 
T neglected ; namely, the organ of sight. The retina, though not 
developed from the epithelial layer of the embryo, is yet pro- 
duced by an outward growth of another portion of the same 
layer. To this, however, I think we should not attach too muai 
