648 



ZOOLOGY. 



Pig. 541. — ^Five schematic figores showing the development of the fcetal e^-mem- 

 branes, where in all except the last the embryo is represented as if seen In longitndinal 

 section. 1. Diagram of egg with zorta pellucida, blastoderm (o, i), germinal disk and) 

 embryo. 2. Egg with the first traces of the yolk-sac (<*) and ammon (la, »«, and am. 



3. E^ with the amnion miitiTig and forming a sac ; the allantois (al) budding ont. 



4. Egg with the viUi of the eerons membrane (sz> : the aUantols larger ; embryo with 

 month and anal opening. 5. 'Egg in which the vascular layer of the allantois lies close 

 to the eerons layer and has grown into the villi of the same, constituting the true 

 chorion ich). Yolk-sac much smaller, about to be drawn into the cavity of the amnion, 



