THE FCETAL ArPENDAGES. 



13 



fluid, tlie Ajnnloii / while the outer layer either disappears or 

 coalesces with the vitelline membrane, to form the Chorioii 

 (Fi-. d). 





'~Avu 



Fig. 8. —Later stasres of the deYelopment of the body of a Fowl than those represented in 

 Fig. 2. — E, embryo at the third day of incubation ; g, heart; h, eye; i, ear; k, visceral 

 arches and clefts ; ^, m, anterior and posterior folds of tiie amnion which have not yet 

 united over the body ; 1, 2, 3, first, second, and thu-d cerebral vesicles ; la, 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 ; Ain, amnion ; All (the allan- 

 tois, hangring down from its pedicle) ; Um, umbihcal vesicle. — G-, uuder-view of the head 

 of the foregoing, the first viaceral ai-ch 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 

 BinMe or double outo'rowth from the under surface of the meso- 



