CH. LIX.] 



THE DECIDUA OF FCETAL MEMBRANES 



887 



d. The urinary and generative organs, except the epithelium of 

 the bladder and urethra. 



The Somatic mesoblast forms the osseous, fibrous, and muscular 

 tissues of the body-wall, including the true skin. 



The Splanchnic mesoblast forms the fibrous and muscular wall of 

 the alimentary canal, the vascular system, and the urine-genital 

 organs. 



3. Prom Hypolblast. a. The epithelium of the alimentary canal 

 from the inner sides of the teeth to the anus, and that of all the 

 glands (including liver and pancreas) which open into this part of 

 the alimentary tube. 



b. The epithelium of the respiratory cavity. 



c. The epithelium of the Eustachian tube and tympanum. 



d. The epithelium lining the vesicles of the thyroid. 



e. The epithelial nests of the thymus. 



/. The epithelium of the bladder and urethra. 



THE DECIDUA AND THE FCETAL MEMBRANES 



When the uterus is ready for the reception of an embryo it is lined 

 by a greatly hyper trophied mucous membrane; this is called the 

 decidua, because, after the delivery of the child, a portion of it comes 

 away from the uterus with the other membranes. 



The ovum has been fertilised 

 in the Fallopian tube, and the 

 embryo, by the time it reaches 

 the uterine cavity, has usually 

 reached the stage of a morula or 

 blastula. It rapidly eats its way 

 into the substance of the decidua 

 which closes over it, obliterating 

 opening through which it 

 and thus the embryo 

 becomes imbedded in the mem- 

 brane, which thereupon becomes 

 separable into three parts. 1. The 

 part between the embryo and the 

 muscular wall of the uterus, the 

 decidua basalis or serotina. 2. The 

 part between the embryo and the 

 uterine cavity, the decidua capsu- 

 laris or reflexa. 3. The remaining 

 part is called the decidua vera. 

 Between the decidua capsularis and the decidua basalis lies the 

 embryo, which speedily becomes differentiated into the foetus and 



the 



FIG. 573. Diagram representing the relation of 

 the developing embryo to the decidua at a very 

 early stage. 1, Uterine muscle ; 2, epi blast 

 of embryo ; 8, inner cell mass of embryo ; 

 4, decidua basalis ; 5, decidua capsularis ; 

 6, decidua vera ; 7, cavity of uterus. 



