The Fetal Membranes 



65 



the ruminant have been destroyed by disease, the former 

 non-placental areas develop adventitious placental struc- 

 tures, but in such cases want their multiple cotyledonal 

 character and take on the simpler structure of a diffuse 

 placenta. 



As soon as the embryo begins to form, its enveloping 

 membranes commence to develop, first the amnion, followed 

 closely by the allantois. The external allantoic wall fuses 

 with the external amniotic layer (false amnnion) to con- 



-FiG. 33 — Fetal and Maternal Placenta of Cow. 



The chorionic or fetal placental tufts are shown above and to the left, 



projecting downward, and to the right, into the placental 



crypts of the uterus. X310 (Pomayer). 



stitute the chorion. In all domestic animals the amnion and 

 allantois each constitutes a sac containing considerable vol- 

 umes of amniotic and allantoic fluids respectively. In uni- 

 para, the allantoic sac normally pushes out to the apices of 

 both uterine horns and to the os uteri internum, thus ac- 

 quiring contact with the uterine mucosa throughout. In 

 ruminants and swine, however, the tips of the allantoic sac 

 early become necrotic in varying degree, as shown in Figs. 

 35 and 36. The necrotic portion retracts at times from the 

 apex of the cornual cavity, leaving the lumen filled with pus 

 and debris. In extreme cases, the entire non-gravid horn of 

 the allantois of the cow becomes necrotic as shown in Plate 



