THE EVIDENCE OF THE SKELETON 13! 
From this tissue all the primitive cariilages which resemble the 
branchial bars are formed, either by the invasion of chondroblasts 
from the surrounding perichondrium, or by the proliferation and 
encapsulation of the cells of the muco-cartilage itself. 
This very distinctive tissue—the muco-cartilage—is of very great 
importance in all questions of the origin of the skeletal tissues. In 
all descriptions of the skeletal tissues it has been practically dis- 
regarded until recent years when, besides my own observations, its 
distribution has been mapped out by Schaffer. Thus Parker, in his 
well-known description of the skeleton of the marsipobranch fishes, 
does not even mention its existence. Its importance is shown by its 
absolute disappearance at 
transformation and its non- 
occurrence in any of the 
higher vertebrates. It is 
entirely confined to the head- 
region, and its distribution 
there is most suggestive, for, 
as will be described fully 
later on, it forms. a skele- 
ton which both in structure 
and position resembles very 
closely the head-shields of 
cephalaspidian fishes, At 
the present part of my argu- 
ment its more immediate Fic. 56.—Sxcrion of Mvuco-CARTILAGE FROM 
interest lies in the method DorsaL Heap-PLaTE oF AMMOCCETES, 
of tracing this tissue. For 
this purpose I made use of the micro-chemical reaction of thionin, 
a dye which, as shown by Hoyer, stains all mucin-containing sub- 
stances a bright purple. Schaffer made use of a corresponding 
basophil stain, hemalum. When stained with thionin, the matrix, 
or ground-substance of the branchial cartilages as well as the matrix 
or semi-fluid substance in which the fibrils of the muco-cartilaginous 
cells are embedded take on a deep purple colour, while the fibrous 
material of the cranial walls and other connective tissue strands, such 
as the perichondrium, are coloured light blue. Muco-cartilage, then, 
may be described as a peculiar form of connective tissue which 
differs from other connective tissue not only in its appearance but in 
