202 COMPARATIVE ZOOLOGY. 



The embryo lies with its face, or ventral surface, tow- 

 ards the yolk, the head and tail curving towards each 



FIG. 1?0. Hen's Egg, more highly developed. The embryo is enveloped by the am- 

 nion, and has the umbilical vessel, or remifcuit of the yolk, hanging from its un- 

 der surface; while the nllantois turns upward, and spreads out over the internal 

 surface of the shell-membrane. (From Daltou's " Physiology.") 



other. Around the embryo on all sides the epiblast and 

 upper layer of the inesoblast 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- wn i c h extends beVOlld the 

 lantois fully formed: 1, umbilical vesi- 



cle, containing the last of the yolk; 2, wall of the embryo proper 



amnion; 3, allautois, on which the fringes . , , 



of the placenta are developing. (From ItttO the Cavity OI the amni- 



Dalton's "Physiology.") Qn ftnd gpreads Qut Qver 



whole inner surface of the shell, so that it partly surrounds 

 both embryo and inner layer of the amnion (amnion prop- 



