48 JOURNAL OF THE [April, 



and the hair of mammals. The scales and rattles of snakes, the 

 tortoise shell of chelonians, claws, prickles, and warts, are all 

 epidermal. 



In this group pigment plays an important role. The chame- 

 leon is a time-honored and well-known example of the change of 

 color to agree with surroundings. In addition to pigment the 

 formation and structure of the scales in relation to the light-rays 

 may also have something to do wdth the general effect. 



Integumentary sense organs are represented in Snakes, and also 

 in Birds, by tactile cells surrounded by connective-tissue pouches, 

 wuth septa separating the individual tactile cells and thus form- 

 ing a tactile corpuscle. 



Dermal ossifications were more developed in ancient reptiles 

 than in those of the present day. Crocodiles, some lizards, and 

 principally the chelonia, still maintain dermal structures. 



D. Birds. 



When we come to examine this group the most characteristic 

 integumentary structures are the feathers, and these \v\\\ there- 

 fore require the greater part of our attention. 



Ordinarily feathers appear to be inserted over the entire body 

 of a bird, but on closer view they will be found, with but few 

 exceptions, in certain regions only, called feather tracts, sepa- 

 rated from each other by naked stretches of skin. These tracts 

 vary in number and position with different genera, as do also the 

 shape and size of the individual feathers. In a general way 

 feathers develop as follows : At the point where one is destined 

 to be formed occurs a slight upheaval of the dermal tissue, fol- 

 lowed by the epidermal layer, and thus creating a papilla. As 

 this papilla grows outward and forms a bluntly-pointed cone 

 (Fig. 3, a), its base sinks gradually inward ; the epidermis imme- 

 diately surrounding follows and forms a pocket around the elon- 

 gated papilla. This papilla is \.h.t feather germ (Fig. 3, b, F), and 

 the pocket constitutes the feather follicle (Fig. 3, b). The papilla 

 is thus made up of the two layers of the epidermis on the outside, 

 acting as a covering for the mass of dermal cells in the interior, 

 Xhtptilp (Fig. 3, b). 



As the feather germ grows the cells of the inner epidermal or 

 Malpighian layer increase rapidly in number, and grow toward 



