The oral cirri of Siluroids Rnd the origin of the head in Vertebrates. 415 



The Siluroids approach near to an autostylic condition, or, to speak 

 more correctly, are little removed from it. 



Histoloîry of the Tentacular Skeleton. 



I venture to give the following sketch of the varieties of cartilage 

 J) resent in the tentacular skeleton. 



Condensed embryonic tissue, known as procartilage, developes in 

 various directions. It njay give rise to an intercellular matrix with 

 a tendency to become refractile. Such a tissue for the sake of com- 

 parison I term soft RIyxinoid tissue (A), inasmuch as it forms the 

 axis of the tentacles of Myxine. The nuclei and protoplasm may 

 disappear, and the intercellular matrix become very hard, as in the 

 hard tissue of Myxine. 



On the other hand the intercellular matrix may become fibrillar 

 and the cells and protoplasm degenerate as in the tentacles of Clarias 

 (B). Or the procartilage may develope into hyaline cartilage (C) or 

 persist to a considerable extent in its embryonic condition (D). The 

 refractile matrix is stained blue by Bleu de Lyon, while the nuclei and 

 protoplasm remain unstained. The core of the tentacle may attain very 

 little development, the tentacle then being very flexible, as in Misgurnus, 

 or on the other hand as in Motella it may be formed by structureless 

 bone with a layer of osteoblasts round it. I have drawn up the accomp- 

 anying scheme of the occurrence of these tissues. A stroke between two 

 letters indicates that the tissue is intermediate between two varieties. 



