CH. LIX.] RESPIRATORY AttD GEXITO-URlNARY ORGANS 



871 



we have the lungs formed: the epithelium lining the air-cells, 

 bronchi, and trachea is derived from the hypoblast and all the rest 

 of the lung-tissue, and of the tubes from the mesoblast. 



The diaphragm is early developed as a partition of mesoblast 



I 



FIG. 670 illustrates the development of the respiratory organs. A is the oesophagus of a chick on the 

 fourth day of incubation, with the rudiments of the trachea on the lung of the left side, viewed 

 laterally; 1, the inferior wall of the (esophagus; 2, the upper portion of the same tube; 3, the 

 rudimentary lung; 4, the stomach ; B is the same object seen from below, so that both lungs are 

 visible. C shows the tongue and respiratory organs of the embryo of a horse; 1, the tongue; 

 2, the larynx ; 3, the trachea ; 4, the lungs, viewed from the upper side. (After Rathke.) 



dividing the original pleuro-peritoneal cavity into thoracic and 

 abdominal serous cavities. 



Development of the Genito-urinary Apparatus. 



In the early stage of the development of the urino-genital organs, 

 the most striking thing seen is their resemblance to the segmental 



FIG. 671. Diagram of transverse section of embryo dogfish. On the right of the middle line, A B, the 

 primitive segmental tube (A') is seen in transverse section ; on the left side a later stage is repre- 

 sented ; it here forms a well-marked projection into the pleuro-peritoneal cavity, and the section is 

 represented as passing through the trumpet-shaped opening of the tube into that cavity (A"). 



organs, or nephridia of worms. The subject was first worked out by 

 Balf our in the elasmobranch fishes ; we may therefore first describe 

 what he found here, and then pass on to what occurs in mammals. 



In the preceding diagram (fig. 671) we have a transverse section 

 through the embryo in which the structures represented will be 



