3S 



BIOLOGY OF THE LABORATORY MOUSE 



the slightly older epithelium nearer the lumen the two layers have split so 

 that two wedgeshaped spaces extend from the lumen between decidua and 

 muscles on each side of the decidual swelling. In course of time the wedges 

 penetrating from the two sides meet ventral to the embryo, thus completing 

 the formation of the new lumen. 



Old uterine lumen 



Decidua basalis 



Mesometrium 



Vascular zone 



Blood vessel 



New uterine 

 lumen 



Giant cell / \ Uterine epithelium 



Maternal blood cells Decidua capsularis 



Fig. 21. — Longitudinal section (partly diagrammatic) of uterus at site of implantation 

 of 8 day 6 hour, 5 somite embryo. Cut parallel to mesometrium. 



Besides the changes in the uterine lumen there are interesting changes in 

 the histology of the decidua. Starting as a relatively homogeneous tissue, 

 dififerent zones differentiate within it, each with its characteristic structure. 

 As many as six zones have been distinguished (7,7,), but for our purposes it 

 will suffice to note no more than three (Fig. 21). Ventrally there is an 

 antimesometrial zone or decidua capsularis charactferized by large bi-, tri- or 

 tetra-nucleate cells.* The individual nuclei in this zone as well as the cells 



I * In the rat this region is characterized by bi-nucleate cells. Krehbiel (33) states 

 that more than two nuclei do not occur. 



