224 EMBRYOLOGICAL TYPES 



been completely absorbed within the body. The beak of the 

 upper jaw bears a sharp projection, the egg-tooth, with which 

 the chick pierces the shell, and soon after, it emerges, having 

 severed its connexion with the allantois. The septum between 

 the auricles of the heart is reformed, and the chick now lives 

 in the same manner as the adult bird. 



Feathers. — Feathers begin developing at about the seventh 

 day of incubation and the first sign of their appearance is in 

 the form of a thickening of the epidermis overlying a con- 

 densation of the dermis, and forming a papilla. The rudiment 

 of the feather soon becomes conical and eventually takes 

 the form of an elongated cylinder. The papilla at its base 

 becomes sunk beneath the general level of the skin forming 

 a follicle. The deepest layer of the epidermis differentiates 

 into a number of longitudinal thickened ridges, two of which 

 will become the rachis, and the remainder will give rise to the 

 barbs which come off from the rachis. At this stage they are 

 still rolled up inside the cylinder and covered by the outer 

 layer of epidermis forming the feather-sheath. The central 

 dermis is nutritive in function, and eventually degenerates. 

 The vane of the feather is formed by the shedding of the sheath, 

 the splitting of the cylinder on the side opposite the rachis, 

 and the flattening out of the barbs which have thus been 

 released, on each side of the rachis. The former cylindrical 

 nature of the feather is betrayed by the presence of a hole at 

 the base of the quill — the inferior umbilicus — and another 

 at the bottom of the vane (between it and the aftershaft), the 

 superior umbilicus. The aftershaft represents the thickenings 

 of deeper layers of the epidermis on the side of the cylinder 

 opposite the rachis, and below the lowermost barbs belonging 

 to the rachis. 



The feathers of the adult bird, pennae, plumulae, and 

 filoplumes, are typically preceded by " nestling-down " in 

 the form of prepennae, preplumulae, and prefiloplumes 

 respectively. There are two generations of prepennae, but 

 in the majority of birds it is the first generation which forms the 

 nestling- down, and the second is reduced. Both generations 

 are present in the penguins. 



