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 monoblast 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 

 which extends beyond the 

 wall of the embryo proper 

 into the cavity of the amni- 

 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- 



Fig. 171. — Mammalian Embryo, with al- 

 lantois fully formed: 1, umbilical vesi- 

 cle, containing the last of the yolk; 2, 

 amnion ; 3, allantois, on which the fringes 

 of the placenta are developing. (From 

 Dalton's "Physiology.") 



