40 



COMPARATIVE ANATOMY 



is the enamel, which is formed as an excretion of the epidermic 

 cells (Fig. 31), while the later formed mesodermal dentinal and 

 bony portions become closely connected with the enamel secondarily. 

 Thus the enamel is the first and originally the only hard substance 

 of the placoid organ. 



In the Holocephali (Chimsera, Callorhynchus) a double row 

 of placoids is developed along parts of the dorsal region in the 

 embryo, but disappear in later stages : in the adult these organs 

 apart from the spine on the anterior margin of the dorsal fin, occur 

 only on the claspers and frontal organ of the male. 1 



Teleostomes. In these Fishes, ossifications in the derm to 

 form bony scales takes place independently of any stimulus from 



FIG. 31. VERTICAL SECTION THROUGH THE SKIN OF AN EMBRYO SHARK. 

 (From Gegenbaur's Comp. Anatomy.} 



C, derm ; c, c, c, d, layers of the derm ; E, epiderm ; e, its layer of columnar 

 cells ; o, enamel layer ; p, dermal papilla. 



the epiderm. Thus the denticle, which in Elasmobranchs is the 

 primary cause of the development of the basal plate, gradually 

 disappears in ontogeny, and the latter is the only part of the 

 placoid organ which remains, its independence being retained 

 in the formation of bony skeletal substance in higher Vertebrates. 

 In Lepidosteus, denticles are still developed in the skin but 

 are quite transitory, and this primitive method of starting the 

 formation of bony tissue is again met with in the Vertebrate series 

 in connection with certain parts of the Amphibian skull ; here 

 certain bones (vomer, palatine, pterygoid, &c.) which originally 

 served as supports for oral teeth, persist even if the teeth dis- 

 appear, as they have become an integral part of the facial 

 skeleton. 



1 In addition to the ordinary placoid scales, larger or smaller spines of a 

 similar structure may become developed in connection with the dorsal fin, around 

 the first cartilaginous ray (e.g. Acanthias, Trygon, Chimsera). In the Saw- 

 fish (Pristis), there is a double row of large denticles on the long rostrum. 



