48 PRINCIPLES OF PHYSIOLOGY 



tympanum, or middle ear ; the lining of the 

 tympanum itself ; the epithelium of the 

 respiratory passages, larynx, trachea, bronchi, 

 and pulmonary air cells ; the epithelium of 

 the thymus and thyroid bodies ; and the 

 epithelium of the urinary bladder and of a 

 portion of the urethra. 



21. From the epithelial portion of the 

 mesoderm, that is the somatopleure portion 

 of the mesoderm, are developed all the 

 voluntary muscles, the epithelium of the 

 Wolffian and Mullerian ducts (primitive excre- 

 tory organs) ; the epithelium of the excretory 

 tubules of the kidneys and Wolffian bodies ; 

 the epithelium of the lining of the body cavity, 

 sometimes called endothelium, the cortex of 

 the suprarenal body, and some of the cells 

 of the ovary and testis. Possibly the germ 

 cells are formed in this layer, but their place 

 of origin has not been conclusively established. 

 Lastly, from the mesenchyme, that is the 

 splanchnopleure layer of the mesoderm that 

 has become associated with the endoderm, 

 we find developed the connective tissues, 

 involuntary muscles, the spleen, the lymphatic 



