846 



IMPREGNATION. 



distinguished as a new temporary lining to the uterus, the so-called decidua. 

 Into this decidua the ovum, on its descent from the Fallopian tube, in which 

 it has undergone developmental changes, extending most probably as far, at 

 least, as the formation of the blastoderm, if not further, is received ; and in 

 this it becomes imbedded, the new growth closing in over it. (Figs. 227, 228.) 

 Meanwhile the rest of the uterine structures, especially the muscular tissue, 



FIG. 227. 



FIG. 228. 



FIG. 227. First Stage of the Formation of the Decidua Reflexa around the Ovum. 

 FIG. 228. More Advanced Stage of Decidua Reflexa. 



become also much enlarged ; as pregnancy advances a large number of new 

 muscular fibres are formed. As the ovum continues to increase in size, it 

 bulges into the cavity of the uterus, carrying with it the portion of the 

 decidua which has closed over it. Henceforward, accordingly, a distinction 

 is made in the now well-developed decidua between the decidua reflexa, or 



FIG. 229. 



Section of a Portion of a Fully Formed Placenta, with the part of the Uterus to which it is 

 attached, a, umbilical cord ; b, b, section of uterus, showing the venous sinuses ; c, c, c, branches, 

 of the umbilical vessels ; d, d, curling arteries of the uterus. 



that part of the membrane which covers the projecting ovum, and the 

 decidua vera, or the rest of the membrane lining the cavity of the uterus, 

 the two being continuous around the base of the projecting ovum. That part 

 of the decidua which intervenes between the ovum and the nearest uterine 



