THE F(ETAL APPENDAGES. 



13 



fluid, the Amnion y while the outer laj'er either disappears or 

 coalesces with the vitelline membrane, to form the Chorion 

 (Fig. 3). 



Flo. 8. —Later stages of the development of the body of a Fowl than those represented U 

 Fig. 2. — E, embryo at the third day of incubation ; ^, heart ; A, eye ; z, ear ; A, viflceral 

 arclies and clefts ; I, m, anterior and posterior folds of the amnion which have not yet 

 united over the body ; 1, 2, 3, first, second, and third cerebral vesicles ; lo, vesicle of the 

 third ventricle. — F, embryo at the fifth day of incubation. The letters as before, except 

 n, o, rudiments of the anterior and posterior extremities ; Am^ amnion ; All (the allan- 

 tois, hanging down from its pedicle) ; Vm, umbilical vesicle. — G, under-vlew of the head 

 of Uie foregoing, the first visceral arch being cut away. 



Thus the amnion encloses the body of the embryo, but not 

 the umbilical sac. At most, as the constricted neck, which 

 unites the umbilical sac with the cavity of the future intestine, 

 becomes narrowed and elongated into the vitelline duct, and 

 as the sac itself diminishes in relative size, the amnion, in- 

 creasing in absolute and relative dimensions, and becoming 

 distended with fluid, is reflected over it (Fig. 1). 



A third foetal appendage, the Allantois, commences as a 

 single or double outgrowth from the under surface of the mcso- 



