564 



INTEGUMENTARY SYSTEM 



Fig. 266. Formation of the scale in Lepisosteus (Lepidosteus) osseiis. (After Nickerson: 

 Bull. Mus. Comp. Zool. at Harvard College, 24.) (A) Section through posterior end 

 of scale of fish, 150 mm. long. (B) Section through posterior end of decalcified scale 

 of fish, 300 mm. long. (C) Section through scale of fish, 300 mm. long. (D) Section 

 showing developing spine. (E) Outlines of scales viewed from surface. (F) Section of 

 scale spine attached to scale. 



scale (fig. 266F). The papillary cells thus become entirely enclosed within 

 the spines of dentine, with the exception of a small canal, leading to the ex- 

 terior, at the base of the spine (fig. 266F). As the dentine-like spine develops, 

 an enamel-like substance is deposited upon its outer surface by the epidermal 

 cells. 



Another characteristic of scale formation in Lepisosteus is the deposition 

 of ganoin upon the outer surface of the scale (fig. 266B, C). This ganoin ap- 

 pears to have many of the characteristics of the enamel. It previously was 

 considered to have been formed by the lower layer of epidermal cells, but 

 Nickerson (1893) concluded that it is of dermal origin. The outer, ganoin- 

 covered surface of the scale eventually lies exposed to the exterior in the adult 

 condition and, therefore, is not covered by epidermal tissue. 



Much of the external surface of the body of the bony ganoid fish, Lepi- 

 sosteus osseus (common garpike), is covered with these plate-like scales, and, 

 consequently, the epidermal layer of the skin tends to be pushed aside by this 

 form of scaly armor. In Amia calva the epithelial (epidermal) covering is 

 retained, and cycloid scales, similar to those of teleosts, are developed. The 

 "ganoid" scales of Amia lack ganoin. They protect the head (fig. 316D). 



