130 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. 
Fic. 55.—A, BrancuiaL CaRTILAGE OF AMMOCGSTES, 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 fibrille, 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 fibrille filled up with a semi-fluid mass. 
