THE INTEGUMENT AND THE EXOSKELETON 47 



C. THE EXOSKELETON IN GENERAL 



The exoskeleton is derived from the skin. It is produced by hardening processes in the 

 «pidermis or the dermis or in both. Exoskeleton derived from the epidermis is spoken of as 

 epidermal; it is produced by the activity of the stratum germinativum and consists of many flat horny 

 ■cells pressed firmly together to make a hard structure. It will be seen that epidermal exoskeleton 

 structures are only special portions of the stratum corneum. Exoskeleton derived from the 

 dermis is spoken of as dermal; it nearly always consists of bone, produced by the mesenchyme cells 

 originating from the dermatome. From the embryological point of view epidermal exoskeleton 

 is of ectodermal origin, while dermal exoskeleton is mesodermal. Epidermal and dermal exoskele- 

 ton are different both morphologically and embryologically. From our definition of homology 

 it follows that exoskeletal structures of epidermal origin are homologous in different animals, 

 and dermal structures are likewise homologous. It will be our purpose to trace the homology 

 of the exoskeleton in the different vertebrate classes. 



In connection with the exoskeleton the following references should be consulted: ,K, pages 

 4i, 43-47; W, pages 76-121; Wd, pages 17-38; CNH, Vol. VII, pages 182-92; R, pages 2-8. 



D. EXOSKELETON OF FISHES 



Most fishes are covered with scales, which are of four kinds. 



1. The placoid scale. — This type of scale occurs in the elasmobranch fishes. 

 It consists of a basal plate carrying a projecting spine. Good examples are obtain- 

 able from the skate. Cut out from the skate a small piece of skin containing one 

 spine. Clean away the skin so as to expose the complete scale. It consists of 

 a toothed basal plate, from which arises a shiny curved spine. The base of the 

 spine is hollow, the cavity being known as the pulp cavity. This cavity can be lo- 

 cated by probing with a needle point in the center of the under side of the basal 

 plate. The shiny coating of the spine is composed of enamel; the basal plate and 

 interior of the spine consist of dentine, a substance similar to bone. Draw the 

 scale. 



To understand the structure of the placoid scale it is necessary to consider its mode of 

 development. Each scale originates from a region of cell multiplication in the dermis. The 

 cells thus formed heap up into a cone, the dermal papilla, which pushes up against the under side 

 of the stratum germinativum. The outer cells of the dermal papilla begin to secrete dentine, 

 and produce the basal plate and the inside of the spine. The stratum germinativum in contact 

 with the dermal papilla secretes on its under surface the substance enamel, 1 which consequently 

 forms a coat over the dentine of the spine. The spine when formed breaks through the sur- 

 face. The interior cells of the dermal papilla do not undergo any change but remain as a soft 

 pulp occupying the pulp cavity in the interior of the spine. From this account it follows that 

 the placoid scale is composed of both epidermal and dermal constituents. The development 

 of the placoid scale is illustrated in Figure 12; also in W, page 80; CNH, Vol. VII, page 

 185; Wd, page 40; R, page 4. 



Draw an imaginary longitudinal section through a placoid scale, coloring ecto- 

 dermal parts blue and mesodermal parts red. 



1 That this substance is really enamel, such as occurs on teeth, has been questioned, some main- 

 taining that it is a kind of dentine (see L, p. 120). We shall, however, here take the usual view of its 

 origin. 



