THE DECIDU^. 



45 



one of the depressions of the uterine lining-, usually near the entrance of the ovi- 

 duct, whereupon the adjacent mucosa undergoes rapid further hypertrophy, which 

 results in the formation of an annular fold surrounding the product of concep- 

 tion. This encircling wall of uterine tissue continues its rapid growth until the 

 embryonic vesicle is entirely enclosed within a capsule of modified mucous mem- 

 brane, known as the decidua reflexa, as distinguished from the decidua vera, the 

 name applied to the general lining of the pregnant uterus. That portion of the 

 uterine mucosa, however, which lies in close apposition to the embryonic vesicle, 

 constituting the outer wall of the decidual sac, is termed the decidua serotina ; later 

 it becomes the maternal part of the placenta. 



Fig. 51. 









Duct of gland 



Spiral portion of 

 gland 



Process of muscular 

 tissue extending be- 

 tween the glands 



Muscular tissue 



L'terine blood- 

 vessel 



Uterine mucous membrane with part of muscular tissue. X 45. 



Our knowledge of the details regarding the encapsulation of the ovum has 

 been materially advanced by the recent observations of Peters, who had the rare 

 good fortune of carefully studying the details of the process at an earlier stage than 

 any hitherto accurately investigated. The results of Peters' s observations lead to a 

 somewhat modified conception of the early phases of the encapsulation of the ovum, 

 as well as shed additional light on some of the vexed problems concerning the details 

 of the formation of the placenta. 



According to these investigations, the embryonic vesicle, on reaching the uterine 



