DEVELOPMENT OF THE SKIN 585 



At about the fifth month, the upper cells of the nail matrix begin to keratinize, 

 and the keratinized cells gradually fuse into the compact nail plate. As new 

 material is added to the nail plate from the cells of the matrix, the distal 

 portion of the plate is pushed progressively toward the end of the digit (fig. 

 274A). Although that portion of the nail field between the terminal end of 

 the digit and the lunula takes no part in the formation of the cornified ma- 

 terial of the nail plate, the underlying dermis below the nail field does form 

 elongated ridges which push upward into the epidermis of the nail field. These 

 ridges secondarily modify the already-formed nail plate by producing fine, 

 longitudinal lines or ridges. 



The claw or nail plate of the cat is compressed laterally to form a narrow, 

 sickle-shaped structure. Three main factors are responsible for this peculiar 

 form of the nail plate in the cat. One factor is the laterally compressed form 

 of the distal phalanx. This condition results in a nail-fold invagination which 

 is laterally compressed. The nail matrix thus is elliptical in shape, dorso- 

 ventrally, instead of flattened as in the human finger. A second factor re- 

 sponsible for the extreme, claw-shaped form of the nail plate in the cat is 

 the more rapid growth in the middorsal portion than in the lateral areas of 

 the nail plate. This discrepancy in growth results in the highly pointed mid- 

 region at the distal end of the nail plate. Ventrally, the two lateral sides of 

 the nail plate tend to approach each other. The area between these two sides 

 is filled with a cornified mass of subungual material. A final factor governing 

 the extreme pointedness of the cat's claw is the fact that the claw-distal- 

 phalanx arrangement, relative to the middle phalanx and tendons, makes the 

 claw retractile when not in use, thus preserving its pointed distal end (fig. 

 274C). 



The dog's claw or nail on the ordinary digits is compressed laterally less 

 than that of the cat, with the result that the subungual cornification is broader 

 and more pronounced and the distal end of the claw not as pointed. However, 

 the claws upon the vestigial first digit, the so-called dewclaws, are pointed and 

 cat-like. The fact that the claw of the dog is non-retractile is a factor in re- 

 ducing its pointedness, for it, unlike the cat's retractile claw, is worn down 

 continually. 



The cloven hoof of the pig or cow is produced by the formation of two 

 nail plates, one dorsal and one ventral, around each of the distal phalanges 

 of the third and fourth digits (fig. 274E). The dorsal nail plate is rounded 

 from side to side and meets the lower nail plate ventrally, with which it 

 fuses along the lateral and distal portions of the lower plate. The unsplit hoof 

 of the horse is produced by a somewhat similar arrangement of dorsal and 

 ventral nail plates around the hoof-shaped phalanx of the third digit (fig. 

 274F, G). A third nail plate or growth center produces the frog or cuneus. 



4) Development of Horns. The horns of cattle arise as two bony out- 

 growths, one on either side of the head, from the area of the parietofrontal 



