13O THE ORIGIN OF VERTEBRATES 



of which the separate cells lie singly. These branchial cartilages are 

 each surrounded by a layer of perichondrium, and in Fig. 55, A, I 

 give a picture of a section of a portion of one of the bars. 



A B 



Fig. 55. — A, Branchial Cartilage of Ammoccetes, stained with Thionin. B, 

 Branchial Cartilage of Limulus, stained with Thionin. 



Hence we see that structurally as well as topographically the 

 branchial bars of Ammoccetes justify their claim to be considered as 

 the origin of the vertebrate cartilaginous framework. 



On the Structure of the Muco-cartilage in Ammoccetes. 



We can, however, go further than this, and ask how this cartilage 

 itself is formed in Ammoccetes ? The answer is most definite, most 

 instructive and suggestive, for in all cases this particular kind of car- 

 tilage is formed from, or at all events in, a peculiar fibrous tissue, 

 which was called by Schneider " Schleim-Knorpel,''' or muco-cartilage, 

 a tissue which is distinguishable from other connective tissues, not 

 only by its structural peculiarities, but also by its strong affinity for 

 all dyes which differentiate mucoid or chondro-mucoid substances. 



This muco-cartilage is thus described by Schneider : — The peri- 

 chondrium in Ammoccetes is not confined to the true cartilaginous 

 structures, but extends itself in the form of thin plates in definite 

 directions. Between these plates of perichondrium a peculiar tissue 

 (Fig. 56) — the muco-cartilage — exists, consisting of fibrillar, whose 

 direction is mainly at right angles to the planes of the perichondrial 

 plates, with star-shaped cells in among them, and with the spaces 

 between the fibrillse filled up with a semi-fluid mass. 



