ratotrichium with calcified outer layer 



cerototrichia 





Tify'ir:VTT¥V-VV-'rxT.Y--^'V'»'»'^.y^'V''i'^v-i(<r»'-T '-y 



epidermis placoid scale 

 muscle/ \ cerototrichia 



B POLYPTERUS OR TELEOST 



lepidotrichium 



D RAJA LINTEA 



cartilaginous radial with calcified core 



Figure 8-46. Frontal section through fins showing interrelationships of ports. (After Jorvik, 1959) 



interface toward the pulp cavity. These He more or less 

 parallel to each other and act as canals for the protoplas- 

 mic fibers of the odontoblasts. The odontoblasts producing 

 the dentine lie on its inner surface, between the dentine 

 and the pulp. The pulp cavity is filled with a gelatinous 

 material in which blood vessels and nerves are found; these 

 e-xtend out through the root of the tooth. 



This band, called the dental lamina (Figure 8-50), retains 

 its connection with the outer epidermis. The dental lamina 

 has bud-like e.xtensions at intervals called the tooth germs. 

 Each tooth germ produces a cup-shaped enamel organ 

 having an inner and outer epithelium and a pulp-filled in- 

 terior. The enamel organ is attached by a strand to the dental 

 lamina. A mesodermal plug extends up into the ventral 



Embryological developmenf Tooth development begins 

 with an epidermal invagination along the margin of the jaw. 



epidermis 



basement membrane 



ratotrichium 

 scleroblast 



actinotrichium 



A 23 



accumulation of sclero- 

 ■■25 " . - blasts of placoid scale 



, ,^,, 'W, S-^ o - ■>_ d. (^ 



' o e^ 





NO generations 

 of cerototrichia 



B 60 mm 



Figure 8-47. Development of cerototrichia in ScyW'ivjn conicu/a. 

 (After Jorvik, 1958, and Goodrich, 1904) 



basal 



c:y ^ membrane 



epidermis 



lepidotrichial thickening of basal membrane 



lepidotrichium separating from basal membrane 



Figure 8-48. Progressively more proximal (toward base) sections 

 through a fin of a So/mo so/or of 21 mm in length. (After Jorvik, 1 959, 

 and Goodrich, 1904) 



244 ■ THE SKIN AND ITS DERIVATIVES 



