DEVELOPMENT. 197 
The septum between the auricles is the last to be finish- 
ed; being closed the moment respiration begins. The 
1a. 168.—Hen’s Egg, more highly developed. The embryo is enveloped by the am- 
nion, and has the umbilical vesicle, or remnant of the yolk, hanging from its un- 
der surface; while the allantois turns upward, and spreads out over the internal 
surface of the shell-membrane. (From Dalton’s ‘‘ Physiology.” 
blood - vessels ramify in all directions through the yolk, 
making it a spongy mass, and all perform the same office ; 
it is not till the fourth or 
fifth day that arteries can 
be distinguished from veins, 
by being thicker and by car- 
rying blood only from the 
heart.’ 
The embryo lies with its 
face, or ventral surface, to- 
ward the yolk, the head and 
tail curving toward each 
other. <A deli sate transpar- Fig. 169. — Mammalian Embryo, with al- 
lantois fully formed: 1, umbilical vesi- 
ent membrane (a part of the cle, containing the last of the yolk; 2, 
amnion ; 3, allantois, on which the fringes 
upper layer.of the, blasto- .o¢ the plicentaiare developing, (Pinu 
derm) rises like a hood over = P#!0™s “Physiology.”) 
the back of the embryo till it forms a closed sac, called 
the amnion. It is filled with a thin liquid, which serves 
