EMBRYOLOGY 211 



and more the appearance of mere branches of the by this time 

 advanced mesenteric vessels. The yellow yolk has become quite 

 fluid, and its bulk has increased OA\ang to its having absorbed much 

 of the rapidly-diminishing white of the egg. During the next days 

 the yolk diminishes rapidly in bulk, it being taken up by the 

 abundantly-developed blood-vessels ; the yolk-sac becomes flaccid, 

 and by the eleventh day is thrown into a series of internal folds. 

 The intestine has by elongation formed a number of convolutions 

 and loops, some of which are hanging down into the somatic stalk, 

 but by the fifteenth day these loops are gradually withdrawn into 

 the cavity of the abdomen, the walls of which have by this time 

 been definitely established like the walls of the chest. The allantois 

 now fills most of the amniotic cavity and lies close under the shell, 

 being separated from the shell membrane only by the thin false 

 amnion and the remains of the vitelline membrane, with which it 

 fuses. When the egg is opened the pulsations of the allantoic 

 arteries at once attract attention. By the nineteenth day the white 

 of the egg has entirely disapj^eared, and the yolk-sac is withdrawn 

 into the abdomen. 



Concerning the changes of the embryo itself it is to be observed, 

 that by the seventh day the head, which is still disproj^ortion- 

 ately large, ceases to grow more rapidly than the body. The 

 tongue appears on the floor of the mouth ; the visceral clefts on the 

 sides of the now more distinctly-marked neck are closed. On the 

 eighth day a white patch of calcareous matter appears on the tip 

 of the nose ; the latter is by the twelfth day transformed into a 

 horny but still soft beak. On the following day nails are visible 

 at the ends of the toes and some of the fingers, and scales on the 

 anterior surface of the feet and toes. Beak, claws, and scales become 

 harder and more horny by the sixteenth day. Feathei's begin to 

 protrude as early as the ninth day from the siu'face of the skin as 

 papillae, especially prominent on the middle line of the back and on 

 the thighs. By the thirteenth day the feathers are distributed over 

 most parts of the body, and acquiring the length of a quarter of an 

 inch apjjear to the naked eye as feathers, their thin horny sheaths 

 allowing their pigmented contents to shew through. The sheaths 

 are not pierced until a day or two before hatching, when some of 

 them are nearly an inch in length. The cartilaginous skeleton is 

 completed by the thirteenth day, and the muscles can be made out 

 with tolerable clearness. Ossification begins already much earlier 

 in various parts of the bones of the limbs, but much of the skeleton 

 remains cartilaginous or imperfectly ossified long after the bird has 

 been hatched. The Avhole embryo changes its position on the 

 fourteenth day so as to lie lengthways in the egg, with its beak 

 touching the shell -membrane where this forms the inner wall of 

 the rapidly-increasing air-chamber at the broad end. On about the 



