202 
COMPARATIVE ZOOLOGY. 
The embryo lies with its face, or ventral surface, tow¬ 
ards the yolk, the head and tail curving towards each 
Fig. 1T0.— 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 mesoblast rise like a hood over the 
back of the embryo till they 
form a closed sac, called the 
amnion. It is filled 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- wlGoh pvtonrU hpxwirl 
lantois fully formed: 1, umbilical vesi- C CXienQS OeyOUCl tile 
cle, containing the last of the yolk; 2 , wall of the embryo proper 
amnion; 3, allantois, on which thefringes . . . " * ^ 
of the placenta are developing. (From into the Cavity of the amni- 
Dalton’s “Physiology.”) j , . . 
on and spreads out over the 
whole inner surface of the shell, so that it partly surrounds 
both embryo and inner layer of the amnion (amnion prop- 
