836 DEVELOPMENT [CH. LIX. 



e. The epithelium of the nasal passages. 



/. The epithelium of the glands opening on the skin and into the 

 mouth, and nasal passages. 



g. The muscular fibres of the sweat-glands. 



2. Prom Mesoblast. a. The skeleton and all the connective 

 tissues of the body. 



b. All the muscles of the body. 



v. The vascular system, including the lymphatics, serous mem- 

 branes, and spleen. 



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 urino-genital 

 organs. 



3. From Hypoblast. a. The epithelium of the alimentary canal 

 from the back of the mouth to the anus, and that of all the glands 

 (including liver and pancreas) which open into this part of the ali- 

 mentary 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. 

 c. The epithelial nests of the thymus. 



/. The epithelium of the bladder and urethra. 



The Decidua and the Foetal Membranes. 



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

 by a greatly hyper trophied mucous membrane, called the decidua, 

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

 from the uterus with the other membranes. 



When the ovum, which has been fertilised in the upper part of the 

 Fallopian tube, reaches the uterine cavity, it is usually in the stage of 

 a morula or blastula. It rapidly eats its way into the substance of 

 the decidua which closes over it, obliterating the opening through 

 which it passed, and thus the ovum becomes embedded in the 

 membrane, which thereupon becomes separable into three parts. 

 1. The part between the ovum and the muscular wall of the uterus, 

 the decidua basalis. 2. The part between the ovum and the uterine 

 cavity, the decidua capsularis or reflexa. 3. The remaining part is 

 called the decidua mm. Between the decidua capsularis and the 

 decidua basalis lies the ovum, which speedily becomes differentiated 

 into embryo, membranes, and appendages. The outermost of the 

 foetal membranes is the chorion ; this is covered with vascular villi, 



