PLACENTA. 



433 



part only forming a discoidal patch; but this grows out all 

 round, and invades the whole of the sub -zonal membrane 

 except at the poles. There is thus formed a broad placental 

 band, where the villi closely unite with the uterine pits 

 specially formed. 



4. Cotyledonary placenta (fig. 206). — This is characterised by 



PlO. 206.— FCETUS OF Shebp. 



AL, AUantois, seen beneath chorion ; Am, amnion ; P, P, placentae on chorion ; C, 

 umbilical cord ; al, al, extremities of allantoidean comua. (Cbauveau.) 



the villi of the placenta being found in round scattered areas, 

 the cotyledons, which fit into correspondiag thick areas studded 

 with pits in the uterus. 



5. In the diffuse placenta (fig. 207) the allantois (al) com- 

 pletely surrounds the embryo (F), and the whole chorion (Ch) 



2 E 



