202 COMPARATIVE ZOOLOGY. 
The embryo lies with its face, or ventral surface, tow¬ 
ards the yolk, the head and tail curving towards each 
Fig. 170.—Hen’s Egg, more highly developed. The embryo is enveloped by the am¬ 
nion, and has the umbilical vessel, 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. ’ ’) 
other. Around the embryo on all sides the epiblast and 
upper layer of the rnesoblast rise like a hood over the 
back of the embryo till they 
form a closed sac, called the 
amnion . It is tilled with a 
thin liquid, which serves to 
protect the embryo. Mean¬ 
while, another important or¬ 
gan is forming on the other 
side. From the hinder por¬ 
tion of the alimentary canal 
an outgrowth is formed 
Fig. 171. 
- Mammalian Embryo, with al- which extends beVOlld the 
lantois fully formed: 1, umbilical vesi- •' 
cle, containing the last of the yolk; 2, wall of the embryo proper 
amnion; 3, allantois, on which the fringes . . . « . 
of the placenta are developing. (From into the Cavity OT t lie a Ill 111- 
Dalton’s “Physiology.”) on ^ ^ over ^ 
whole inner surface of the shell, so that it partly surrounds 
both embryo and inner layer of the amnion (amnionprop- 
