82 ANIMAL PHYSIOLOGY. 
The decidua serotina and reflexa thus become the outermost 
of all the coverings of the ovum. These and some other devel- 
opments are figured below. It is to be remembered, however, 
that they are highly diagrammatic, and represent a mixture 
Fia. 90.—Series of diagrams representing the relations of the decidua to the ovum. at different 
periods, in the human subject. The decidua are dark, the ovum shaded transversely. In 
4 and 5 the chorionic vascular processes are figured (after Dalton). 1. Ovum resting on 
the decidua serotina ; 2. Decidua reflexa growing round the ovum; 3. Completion of the 
decidua around the ovum ; 4. Villi, growing out all around the chorion ; 5. The villi, spe- 
cially developed at the site of the future placenta, having atrophied elsewhere. 
of inferences based, some of them, on actual observation and 
others on analogy, etc. 
The figures will convey some information, though appear- 
ances in all such cases must be interpreted cautiously for the 
reasons already mentioned. 
During the first fourteen days villi appear over the whole 
surface of the ovum; about this fact there is no doubt. At 
the end of -the first month of foetal life, a complete chorion 
has been formed, owing, it would seem, to the growth of the 
allantois (its mesoblast only) beneath the whole surface of the 
subzonal membrane. From the chorionic surface vascular pro- 
cesses clothed with epithelium project like the plush of velvet. 
t 
