52 LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



external to the present vertebral and costal plates, and that these plates subsequently dissap- 

 peared. On both points of view the nuchal, marginal, postneural, and pygal plates are dermal 

 and part of the exoskeleton. 



b) The plastron: The plastron, like the carapace, consists of a set of horny 

 epidermal scutes covering bony dermal plates. Study the external (ventral) 

 surface of the plastron. It is covered by six pairs of scutes named from in 

 front backward: gular, humeral, pectoral, abdominal, femoral, and anal. Irregular 

 inframarginal scutes cover the bridges. Study the internal (dorsal) surface, 

 noting the large bony plates united by jagged sutures, which compose it. The 

 small anterior pair of plates are named epiplastra. Between them is a single 

 median plate with a posteriorly projecting point, named the ento plastron or inter- 

 clavicle. Behind these are three pairs of large squarish plates named the hyo- 

 plastra, hypoplasia, and xiphiplastra, the first named being most anterior. Draw 

 the plastron, showing outlines of scutes and plates. 



c) Other exoskelelal structures: The exoskeleton also includes the claws 

 and the horny beaks which incase the. jaws. Examine these beaks in a demon- 

 stration specimen in which they have been loosened from the underlying bones. 

 Turtles also possess scales or thickened scalelike areas on the legs and tail, and 

 in some cases enlarged scutes over the head. All of these structures are of epi- 

 dermal origin, consisting of special portions of the stratum corneum. 



G. EXOSKELETON OF BIRDS 



Birds are clothed in an exoskeleton consisting of feathers on the greater part 

 of the body, scales and claws on the feet, and horny beaks. All of these struc- 

 tures are of epidermal origin, formed from the stratum corneum. There are no 

 dermal elements of the exoskeleton in birds. The feathers of birds are their 

 characteristic features and will be studied in detail. There are three kinds of 

 feathers — down feathers, contour feathers, and hair feathers or filoplumes. 



i. Structure of the down feather. — Down feathers or plumulae constitute 

 the fluffy covering of young birds and also occur in adult birds between the bases 

 of the contour feathers. Obtain a down feather or a prepared slide of one. 

 Identify the short stem or quill, the soft rays or barbs which spring in a circle 

 from the top of the quill, and the minute side rays or barbules on the barbs. Draw. 



2. Development of the down feather. — A down feather arises from a papilla of the skin, 

 the feather papilla, consisting of a dermal core, the pulp, covered by the epidermis. Later this 

 papilla sinks into a pit in the skin, called the feather fdlirfc. The stratum germinativum of 

 the papilla begins to proliferate, forming a number of longitudinal columns which project into 

 the pulp (see Fig. 14). These columns eventually separate, each being a barb, composed of 

 cornified cells. The original stratum corneum covers the barbs like a sheath, which is called 

 the periderm; it splits open and is shed, releasing the barbs. During this process the whole 

 papilla has been elongating and comes to project above the surface. The lower part of the 

 papilla does not split into barbs but remains undivided as the quill, within which the pulp dries 



