MEMBRANE DECIDUJS. 373 



it shuts off a portion of the allantois contained in the abdom- 

 inal cavity, which becomes the urinary bladder ; but there is 

 a temporary communication between the internal portion and 

 the lower portion of the cord, which is called the urachus. 

 This is generally obliterated before birth, and is reduced to 

 the condition of an impervious cord ; but it may persist dur- 

 ing the whole of intra-uterine life, in the form of a narrow 

 canal, extending from the bladder to the umbilicus, which is 

 closed soon after birth. 



Membrance Deciduce. In addition to the two membranes 

 connected with the foetus, there are two membranes formed 

 from the mucous membrane of the uterus, which are derived 

 from the mother, and which serve still farther to protect the 

 ovum. The chorion, as we have just seen, is for the protec- 

 tion of the foetus ; but a portion of this membrane, about 

 one-third of its surface, becomes closely united with a corre- 

 sponding portion of the uterine mucous membrane, to form 

 the placenta. This organ, which serves for the nutrition of 

 the foetus, will be described by itself ; but, before we can 

 thoroughly comprehend its structure and the process of its 

 development, we must study carefully the formation of the 

 membranse deciduee. 1 



As the fecundated ovum descends into the uterus, it is 

 usually invested with a shaggy covering, which is either the 

 permanent chorion or one of the membranes which invests 

 the ovum previous to the complete development of the allan- 

 tois. At this time, the mucous membrane of the uterus has 

 undergone certain changes by which it is prepared for the re- 

 ception of the ovum. The changes which this membrane 



1 The first correct account of the decidua was given by William Hunter, who 

 recognized this as a modification of the uterine mucous membrane, and gave it 

 the name it now bears. The engravings illustrating the anatomy of the gravid 

 uterus, with the membranes, were published in 17 74. A full account of the 

 decidua is in a posthumous publication, edited by Dr. Rigby. (HUNTER, An 

 Anatomical Description of the Human Gravid Uterus and its Contents, London, 

 1843, p. 46, et seq.) 



