THE EVIDENCE OF THE SKELETON 131 



From this tissue all the primitive cartilages 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 \PWcH i ' Sv fH\J ■ 4 tot^'l 



entirely confined to the head- ^ WU \, ) lli-r ','? f P^m m>-^ 



ton «■ 



region, and its distribution r'^'i w<*^f \P s' : 



there is most suggestive, for, 



scr 

 later on, it forms a skele 



Tmmwm 



ton which both in structure ,' V 1 'IfjJnJ Vf 



\ S iw» - n^.^ . ., (I ' .W— i ; 





) 



is will be described fully *>»?$:,*/ 

 Later on, it forms a skele- 

 ton which both in structure 

 and position resembles very 



closely the head-shields of ~W~JXZ- i *& 3i ~^^' '^" 

 cephalaspidian fishes. At '"- — ; C " N \ ( 



the present part of my argu- "~-~- — J v . 



ment its more immediate Pig. 56.— Section of Muco-cartilage from 

 interest lies in the method Dorsal Head-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, hsemalum. 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 



