302 Vertebrate Embryology 



implantation has taken place, is partially un- 

 covered ; but with other mammals, including 



cT?T -4H-P 



^g^fijfl 8 ! 

 ii s 



FIG. 97. HUMAN EMBRYO. AGE SEVEN WEEKS. (From 

 Kollmann.) 



cf, chorion frondosum. <:/, chorion laeve. 



man, the uterine mucosa grows over the ovum 

 until it is entirely covered. The human ovum 

 is normally implanted in the dorsal wall of the 

 uterus. 



When pregnancy occurs, a marked change 

 takes place in the mucous lining of the uterus, 

 consisting chiefly in the degeneration of the 

 glandular epithelium and a conversion of a 

 great number of the connective-tissue cells into 

 the large, so-called decidual cells. This trans- 

 formed mucosa is now called the decidua or 



