THE PELVIC GIRDLE AND HIND-LIMB. 335 



6. The Hind-limb or Leg. 



The femur, and the tibia and fibula, present no special 

 points of interest in their development. 



The tarsus consists, on the seventh day, of a proximal row of 

 two tarsal cartilages, of which one represents the tibiale and 

 intermedium, and the other the fibulare of the typical tarsus; 

 and a single distal cartilage, which shows indications of its for- 

 mation from three centres. At a later stage the proximal tarsal 

 cartilages fuse together ; and about the fourteenth or fifteenth 

 day the proximal tarsals fuse with the distal end of the tibia to 

 form the tibio-tarsus, while the distal tarsal cartilage fuses with 

 the metatarsals to form the tarso-metatarsus. 



In the pes, the first digit, or hallux, is represented by a short 

 metatarsal, of which the proximal end is never present ; and two 

 phalanges. The second, third, and fourth digits are approximately 

 equal in size, having well-formed metatarsals, and three, four, 

 and five phalanges respectively. The fifth digit is represented 

 by a small nodule of cartilage on the outer, or fibular, side of the 

 proximal end of the fourth metatarsal. 



The three fully-developed metatarsals, i.e. the second, third, 

 and fourth, remain distinct, though closely apposed, until about 

 the beginning of the third week of incubation, when they fuse 

 with one another and with the distal tarsal cartilage. 



DEVELOPMENT OF THE FEATHERS. 



The feathers are formed by special modification of the epi- 

 dermal coverings of papillge, which appear as projections of the 

 skin about the eighth day of incubation. 



Each of the permanent feathers is preceded by an embryonic 

 or down feather, the mode of development of which is as follows. 

 About the eighth day small conical projections of the skin appear, 

 the feather papillae, each consisting of a central core of vas- 

 cular connective tissue, covered by a cap of epidermis. As the 

 papillas increase in height, their bases become depressed below 

 the general surface of the skin. This depression is most marked 

 on the surface of the papillae towards the tail end of the embryo, 

 and gives rise to the characteristic backward slant of the feathers. 

 The epidermal cap of each papilla consists of a superficial, 

 or epitrichial, layer of flattened pavement cells, and a deeper or 



