202 COMPARATIVE ZOOLOGY. 



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

 ards tlie yolk, the head and tail curving towards each 



Pio. iro 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 

 BUi-face of the shell-membrane. (From Daltou'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 tlie 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 



Fio. m.- Mammalian Embryo, with al- which extCnds bcVOnd the 



lantois fully formed: 1, umbilical vesi- •' 



cie, contaiuiug the last of the yolk ; 2, wall of the embryo proper 



amnion; 3,allaiitais,ouwhichthefringes • . .i -l £ i.1. 



of the placenta me developing. (Prom mtO tllC cavity 01 the amni- 



Dalton's "Physiology.") ^^ ^^^ Spreads OUt oVcr the 



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

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



