256 MICROSCOPIC ANATOMY OF THE ORGANS. 



developes, changes take place resembling those of menstruation. 

 At the end of the fifth month the mucosa has become more 

 than 1 cm. thick. This is due, in the first place, to the dilata- 

 tion of the blood-vessels and the thickening of their walls, and 

 also to the increase in length of the gland tobules. The latter 

 become corkscrew-shaped or tortuous in their course. The 

 tunica propria increases in its superficial part, so that there is a 

 firm connective tissue between the necks of the glands. In 

 consequence of this, the whole mucosa can be divided into two 

 zones, a superficial compact layer, and a deep spongy layer. 

 From these connective-tissue cells, the so-called decidual cells, 

 arise. These are very large (30-100 u\ round or polygonal 

 cells somewhat resembling epithelial elements. Each cell pos- 

 sesses usually only one nucleus, but some may contain as many 

 as forty nuclei (giant cells) (Fig. 192). These will be spoken 

 of later. The decidual cells are developed especially in the 

 compact layer, where the glands have a straight course and are 

 separated by much connective tissue. In the spongy layer the 

 cells form narrow septa between the saccular ends of the 

 glands. 



The surface epithelium vanishes entirely, while the gland 

 cells increase in number and become flattened to accommodate 

 themselves to the widened gland luraina. 



In the second half of pregnancy changes in the decidua 

 vera occur, which are due mainly to pressure exerted by the 

 growing foetus and the increasing amniotic fluid. The decidua 

 becomes gradually thinner, so that at the end of pregnancy it 

 is only 2 mm. thick. The glandular epithelium degenerates, 

 with the exception of that in the ends of the glands which 

 rest on the muscle. This remains, and is the basis of the epi- 

 thelial regeneration which takes place after pregnancy. The 

 gland necks in the compact layer become obliterated and disap- 

 pear about the middle of pregnancy. The gland lumina in the 

 spongy layer, on the contrary, are converted into spaces which 

 lie parallel to the surface of the uterine wall. 



The decidua reflexa (capsularis) has originally the same 

 structure as the decidua vera ; but during the first months of 



