130 



THE ORIGIN OF VERTEBRATES 



star-like cells, however, are not arranged concentrically around these 

 spaces, as in true Haversian canals. 



This structure is therefore a calcareous infiltration of a tissue 

 with cells in it. Where is there anything like it ? 



As soon as I saw Rohon's picture (Fig. 130), I was astounded 

 at its startling resemblance to the structure of muco-cartilage as is 

 seen in Fig. 131, taken from Ammocoetes. If such muco-cartilage 

 were infiltrated with lime salts, then the muco-cartilaginous skeleton 

 of Ammocoetes would be preserved in the fossil condition, and be 

 comparable with that of Cephalaspis, etc. 



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Fig. 130.— Section of a Head- 

 Plate OF A CEFHALASPID. 

 (From Rohon.) 



Fig. 131. — Section of Muco- 

 Caetilage from Doesal 

 Head-Plate of Ammoccetes. 



The whole structure is clearly remarkably like Eohon's picture of 

 a section of the head-plate of a Cephalaspid (Fig. 130). In the latter 

 case the matrix contains calcium salts, in the former it is composed 

 of the peculiar homogeneous mucoid tissue which stains so charac- 

 teristically with thionin. With respect to this calcification, it is 

 instructive to recall the calcification in the interior of the branchial 

 cartilages of Limulus, as described in Chapter III., for this example 

 shows how easy it is to obtain a calcification in this chondro-mucoid 

 material. With respect to the medullary spaces and smaller spaces in 

 this tissue, as described by Eohon, I would venture to suggest that they 

 need not all necessarily indicate blood-vessels, for similar spaces would 

 appear in the head-shield of Ammocoetes if its muco-cartilage alone 



