NOTES ON A TRACT OF MODIFIED ECTODERM. 99 
thin plate or system of plates, separated from the body- 
wall by short muscular fibres, which, however, do not 
appear to pass through the plates (P1.IV., fig. 1). The sub- 
epithelial tissue stains readily with either picro or alum 
carmine. In the arms this tissue is usually spoken of as 
the cartilaginous skeleton ; it forms the great mass of the 
arms; it is pierced with the canals already referred to, 
and in some parts, notably in the region of the mouth, it 
may almost be said to be riddled with holes in addition to 
those spoken of as the cesophageal lacune of the arms; 
these I believe to be simply due to the method of pre- 
paration, and probably represent cavities resulting from 
decalcification. I am led to suppose that this is so from 
the fact that these tiny holes are not lined with an 
epithelium, as all the larger ones are. With the exception 
of the arms, the sub-epithelial tissue appears to be every- 
where perfectly homogeneous ; so far as I have observed 
there are neither fibrille nor cells to be seen anywhere. 
In the arms, however, there is apparently an exception to 
this rule; fibrille and cells are abundant, and the fibrils 
often appear to be connected with the cells, and to follow 
certain more or less definite paths (see PI]. IIL., figs. 2 and 3). 
At first I imagined that these cells were of the nature of 
cartilage corpuscles, and that the fibrille were in some way 
connected with them; this explanation, however, IT aban- 
doned, and from the fact that the fibrille were constantly 
seen to abut upon a region of modified epithelium placed 
upon both the inner and outer sides of the base of the 
brachial fold, I was inclined to look upon them as nervous 
in nature, and when I found that recent observations 
had traced the brachial nerve plexus to a similar position, 
I did not hesitate to consider them to be the same 
structures. The question then naturally arises, are these 
fibres actually connected with the tract of modified epithe- 
