INTRODUCTION 



affecting most its basal (germinal) layer, these elevations being 

 occupied by mesenchyme (fig. 4)- The cells of these protrusions 

 now secrete an organic substance (ossein) on their outer surface, 

 which is impregnated with calcareous salts, thus forming the basal 

 substance (dentine) of the scale. From this as a centre, the deposi- 

 tion of dentine extends laterally on the corium as far as the modified 

 mesenchyme cells occur. The result is a basal plate formed by the 

 distal parts of the condensed mesenchyme, with a central spine 

 extending from the centre into the elevated part of the ectoderm. 

 Basal plate and spine differ in that no cells are included in the dentine 

 of the spine, while they are embedded in the substance of the basal 

 plate. The cells of the germinal layer of the epidermis become 

 columnar, forming the enamel organ which secretes from its deeper 



Fig. 4. — Sections of developing scales of Acanlhias. c, stratum corneum; ee, 

 enamel organ secreting enamel (densely stippled); d, dentine; w, rm. germinal layer; p, 

 pulp. 



surface a very dense calcareous layer (enamel) which covers the 

 spine, but not the basal plate. A placoid scale is therefore a product 

 of both mesenchyme and ectoderm. By continued growth the tip 

 of the spine protrudes through the skin, while the interior of the spine 

 has a pulp cavity, occupied with the mesenchymatous 'pulp' together 

 with minute blood vessels and nerves. 



A union of several such scales would result in an osseous plate, 

 which, with the loss of spines and enamel, would be very like the 

 bony plates on the heads of many primitive fishes and would account 

 for the membrane bones soon to be described. 



Bone. — Two types of bone are recognized, differing in develop- 

 ment, although indistinguishable in the adult, except by following 

 their history. One is the dermal or membrane bone which arises 

 as a direct ossification of mesenchyme, without any cartilage in its 



