FCETAL MEMBRANES. 587 



The Decidua Parietalis. The changes in the uterine mucosa which 

 result in the formation of the decidua parietalis are similar to, though more 

 extensive than, the changes which take place during the earlier stages of men- 

 struation. There is congestion of the stroma with proliferation of the con- 

 nective tissue elements and increase in the length, breadth and tortuosity of the 

 glands. These changes result as in menstruation in thickening of the mucosa 

 so that at the height of its development the decidua parietalis has a thickness of 

 . about i cm. It extends to the internal os where it ends abruptly, there being no 

 decidua formed in the cervix. 



In the superficial part of the mucosa the glands wholly or almost wholly 

 disappear and their place is taken by the proliferating connective tissue of the 

 stroma. The result is a layer of comparatively dense connective tissue the 

 compact layer. Beneath this layer are found remains of the uterine glands in 

 the shape of widely open, somewhat tortuous spaces which extend for the most 

 part parallel to the muscularis. Some of these glandular remains retain part 

 of their epithelium. Lying in the proliferating stroma, these spaces give to this 

 layer the structure which has led to its being designated the spongy layer. 



During the latter half of pregnancy the decidua parietalis becomes greatly 

 thinned, due apparently to pressure from the growing embryo with its mem- 

 branes. With this thinning, the few remaining glands of the compact layer 

 disappear. The character of the spongy layer changes, the glands collapsing or 

 being reduced to elongated, narrow spaces parallel to the muscularis. The 

 entire tissue also becomes much less vascular than in early pregnancy. 



If the foetal membranes are in situ the compact layer is in contact with the 

 ectodermic (epithelial) layer of the chorion. Next to this lies the mesodermic 

 (connective tissue) layer of the chorion. Delicate adhesions connect the 

 mesodermic tissue of the chorion with the mesodermic layer of the amnion. 

 Covering the latter is the amniotic ectoderm (epithelium). 



The Decidua Capsularis. Early in its development this has essentially 

 the same structure as the decidua parietalis. Its older or more common name, 

 decidua reflexa, indicates the earlier idea that this portion of the decidua repre- 

 sents a growing around or reflection of the uterine mucosa upon the attached 

 ovum. Peters, after examining the very early ovum which bears his name, 

 came to the apparently warranted conclusion that instead of the uterine mucosa 

 growing out around the ovum, the ovum buries itself in the mucosa, and that by 

 the time the ovum had reached the size of the one he examined (i mm.), it was 

 almost entirely covered over by the mucosa (Fig. 74). See also Fig. 493. In 

 Peters' ovum a coagulum consisting of blood cells, other cast off cells and 

 fibrin marked the point at which the ovum probably entered the stroma. 

 Later this is replaced by connective tissue and for a considerable time the point 

 is marked by an area of scar tissue. 



