578 INTEGUMENTARY SYSTEM 



nearest the proximally placed barbs. As a result, a series of horny, hollow 

 cells are formed within the base of the developing feather shaft. This hollow, 

 basal end of the feather shaft forms the quill or calamus. The quill has a 

 proximal umbilicus or opening through which the dermal pulp extends into 

 the interior of the quill in the intact feather (fig. 27 lA). A distal umbilicus, 

 from which the after feather emerges, may also be present in some feathers 

 at the point where the ventral groove of the shaft meets the upper end of the 

 quill. 



3) Formation of the After Feather. The after feather emerges from the 

 upper end of the quill of the contour feather. It is well developed in the un- 

 specialized, contour feather but may be absent or represented merely by a 

 few barbs in flight and tail feathers of the fowl (Lillie and Juhn, '38). For a 

 description of the after feather and its distribution in birds, reference may be 

 made to Chandler ('16). 



As observed above, when the rachis or shaft reaches a certain size, the de- 

 velopment of barbs tends to be suppressed. A stage is reached ultimately 

 when the barbs are irregular and not well formed. Consequently, the barbs 

 near the quill lose all tendency to form a vane and are placed iii an irregular 

 fashion along the shaft. As this distortion of barb development occurs dorsally, 

 some of the developing barbs on the ventral side of the enlarged shaft become 

 physiologically and morphologically isolated from those which are moving 

 dorsad in the normal fashion along the collar. As a result, they remain on 

 the ventral surface and, in this position, they endeavor to form a twin feather. 

 In doing so, they become attached in their isolated position to the ventral 

 aspect of the forming quill. The superior umbilicus marks this point of 

 attachment. 



The degree of development of the after feather varies from the presence 

 of a few barbs to a condition where a well-formed, miniature, secondary feather 

 is developed. The secondary or after feather in this condition possesses a 

 secondary rachis or aftershaft, known as the hyporachis, and is attached to 

 the main rachis at the superior umbilicus. 



4) Development of the Later Down and Filoplumous Feathers. The de- 

 velopment of the later down or undercoat feather is similar to that of the 

 nestling down feather, with the exception that a basal shaft or quill is formed 

 to which the barbs become attached at the distal end of the quill. In the for- 

 mation of the hair feather or filoplume, an elongated shaft of small diameter 

 is formed to which a few small barbs may be attached at the distal end. 



5. Mammals 



a. Characteristics of the Mammalian Skin 



The adult skin of mammals is characterized by a highly cornified, outer 

 layer of the epidermis, together with the presence of numerous glands and 

 hair. Hair, a distinguishing feature of the mammalian skin, is present in all 



